Roles of Mitochondrial 17β-Hydroxysteroid Dehydrogenase Type 10 in Alzheimer’s Disease

Abstract

17β-Hydroxysteroid dehydrogenase type 10 is a multifunctional, homotetrameric, mitochondrial protein encoded by the HSD17B10 gene at Xp 11.2. This protein, 17β-HSD10, is overexpressed in brain cells of Alzheimer’s disease (AD) patients. It was reported to be involved in AD pathogenesis as the endoplasmic reticulum-associated amyloid-β binding protein (ERAB) and as amyloid-β binding alcohol dehydrogenase (ABAD). However, the exaggerated catalytic efficiencies for ERAB/ABAD in these reports necessitated the re-characterization of the catalytic functions of this brain enzyme. In addition to isoleucine metabolism, 17β-HSD10 is also responsible for the mitochondrial metabolism of neurosteroids such as 5α-androstane-3α,17β-diol and 17β-estradiol. These neurosteroids are inactivated by the oxidation catalyzed by 17β-HSD10. Since neurosteroid homeostasis is presumably essential for cognitive function, analysis of the impact of 17β-HSD10 and its inhibitor, amyloid-β peptide (Aβ), on the metabolism of neuroactive steroids offers a new approach to AD pathogenesis.

Keywords

Data integrity mitochondria neurosteroid short chain 3-hydroxyacyl-CoA dehydrogenase steroidogenesis

INTRODUCTION

17β-Hydroxysteroid dehydrogenase type 10 (17β-HSD10) is a multifunctional homotetrameric mitochondrial protein that performs a variety of critical functions. It acts autonomously and also binds to other proteins such as estrogen receptor alpha and RNA (guanine-9-)methyltransferase domain containing 1 to perform different physiological functions [1 –11]. Unfortunately, progress in this research area has been impeded by misinformation and misleading acronyms: endoplasmic reticulum (ER)-associated amyloid-β binding protein (ERAB) and amyloid-β binding alcohol dehydrogenase (ABAD) [12 –17]. Although the Human Genome Organization (HUGO) designated the gene HSD17B10 [3], many other confusing names appear in the literature (Table 1).

Table 1

Alternative designations of human HSD17B10 gene product^*

Year	Accession number		Name	Acronym	Comments	Ref
	cDNA	Gene
1997	U96132	n/a	Endoplasmic reticulum-associated Aβ-binding protein	ERAB	MW = 27 kDa, composed of 262 amino acid residues and associated with ER	[12]
	AF035555	AF037438	Short-chain 3-hydroxyacyl-CoA dehydrogenase	SCHAD	MW = 108 kDa, Homotetrameric enzyme exhibits HAD activity and proposed to reside in mitochondria	[6]
1999			Amyloid β-peptide binding alcohol dehydrogenase	ABAD	Substitution for ERAB to convey incongruous data of generalized alcohol dehydrogenase (C2-C10) activities	[13] [14]
			17β-Hydroxysteroid dehydrogenase	17β-HSD	Mitochondrial, multifunctional protein inactivates 17β-estradiol to estrone	[7]
2001	OMIM 300256, 17β- Hydroxysteroid dehydrogenase XK^*		Type 10 17β-Hydroxysteroid dehydrogenase^*	17β-HSD 10^*	Identification of its N-terminal mitochondrial targeting signal	[8]
2003			2-methyl-3-hydroxyacyl-CoA dehydrogenase	MHBD	Essential for the isoleucine metabolism	[18]
2007	NM_004493, Gene symbol: HSD17 B10^*		3-Hydroxyacyl-CoA dehydrogenase type 2	HADH2^a	A silent mutation was identified in MRXS10 patients	[3] [19]
2008			Mitochondrial RNase P protein 2	MRPP2	A component of RNA-free RNase P	[10]

*The officially designated name of this gene and its gene product is highlighted. ^a17β-HSD10 was also known as HADH2 which was classified as SDR5C1 by B. Persson et al. [49].

17β-HSDs reversibly catalyze the conversion of a hydroxyl group to a ketone group at position C17 and orientation β of a steroid ring, and thus these enzymes are involved in the metabolism of steroid hormones or neurosteroids in the nervous system [5, 11]. Among this class of enzymes, 17β-HSD10 is the only one localized in mitochondria [7, 8]. In contrast, types 2, 3, 6, 7, and 12 are localized in the ER while types 1 and 5 are in the cytosol and type 4 is in the peroxisome [20]. The designation ERAB [12, 13] explicitly indicated the association of this protein with the ER. However, a distortion (see Fig. 5 of [8] or Supplementary Material 2a Bottom) of the conventional subcellular fractionation procedure [21] was responsible for indicating its association with ER (see Fig. 7 of [13] or Supplementary Material 2a Top). It was pointed out in the correspondence from Prof. C. de Duve (Supplementary Material 1) that cytochrome C oxidase rather than cytochrome C should have been used as the mitochondrial marker in this assay. Unfortunately, several reports [6 –8] and the Nobel Laureate’s opinion failed to lead to publication of a corrigendum [12 –14]. It had been predicted [6] and then demonstrated that there is a N-terminal mitochondrial targeting signal in 17β-HSD10/ERAB [8]. The localization of 17β-HSD10 to the mitochondrion [7, 8] was subsequently re-reported for this protein, referred to as ABAD (Aβ-binding alcohol dehydrogenase), in Science without reference to the earlier publications. There is as yet no explanation for the discrepancy between immunohistological micrographs stained with guinea pig or mouse anti-ABAD [14] and those stained with rabbit anti-ERAB [12, 13] (see Fig. 4 of [12] or Supplementary Material 2b and Figs. 6 & 8 of [13] or Supplementary Material 2c), even though ABAD and ERAB are both alternate names for 17β-HSD10 (Table 1). Although these antibodies [12 –14] are still commercially available, there has been no published resolution of this question.

17β-Estradiol and dihydrotestosterone (DHT) are the most potent estrogenic and androgenic hormones, respectively. 5α-Androstane-3α,17β-diol (5α-Adiol) is a very weak androgen that occurs in brain and has anti-depressant, anxiolytic, and anticonvulsant properties [22, 23]. Here we show how mitochondrial 17β-HSD10 mediates the pre-receptor control of neurosteroid metabolism by catalyzing the removal of 5α-Adiol and the oxidative inactivation of 17β-estradiol. The most important contribution of 17β-HSD10 to Alzheimer’s disease (AD) may be that its overexpression in brains of AD patients [12, 24] disturbs the homeostasis of neurosteroid metabolism [5, 11]. This is a fertile topic for future research into AD pathogenesis.

MATERIALS AND METHODS

Materials

17β-Estradiol, estrone, 5α-androstane-3α,17β-diol and dihydrotestosterone, NAD⁺, NADH and all other standard biochemicals were obtained from Sigma.

Protein expression and purification

The plasmid pSBET-6xHis-HSD10 was transformed into E. coli BL21(DE3)pLysS (Novagen) by the one step transformation method [25]. The transformants were induced by 1 mM IPTG for 6 h. The preparation of cell extracts and the purification of 6xHis-HSD10 were accomplished by use of a Ni-NTA Fast Start kit (Qiagen) as described previously [1].

Protein assay

Protein concentrations were determined by the Bio-Rad Protein Assay kit per the manufacturer’s instruction.

Enzyme assays

17β-HSD10 activity was assayed at 25°C by measuring the absorbance changes at 340 nm as a function of time using a continuously recording spectrophotometer (Hitachi U-3010). A standard assay mixture for dehydrogenation reactions contained 0.1 M potassium phosphate, fatty acid-free bovine serum albumin (0.1 mg/ml), 100 μM steroid substrate (added in 8 μl of ethanol), 2 mM NAD⁺ and appropriate quantities of enzyme. When the reduction of steroid was measured, 100 μM NADH was substituted for NAD⁺ as the coenzyme. The standard enzyme assay method was designated as Procedure A. Kinetic parameters of 6xHis-HSD10 were determined with steroids as variable substrates at several fixed coenzyme concentrations and at different pH value. In general, pH 7.4 and 8.0 are applied for androgenic and estrogenic substrate, respectively. When the oxidation of 17β-estradiol catalyzed by the dehydrogenase was studied, the enzyme assay was also performed by following another published procedure [13] as the Procedure B. 17β-estradiol (61 μM) and NAD⁺ (0.4 mM) in 20 mM sodium phosphate (pH 8.9) were reacted at 25°C under the catalysis of the dehydrogenase (150 μg/ml) for 2 h. If the absorbance at 340 nm was monitored every 5 min for 2 h (13) or shortened to a total of ½ h in some cases of the present study, the method was designated as Procedure B1. If A₃₄₀ recorded by use of a continuously recording spectrophotometer (Hitachi U-3010) after a few seconds of instrumental operation delay, the method was designated as Procedure B2. The molar extinction coefficient used for calculating rates is 6,220 M^–1cm^–1. Kinetic data were analyzed by the Leonora computer program [26] and/or the Lineweaver-Burk plot with linear regression [27]. A unit of activity is defined as the amount of enzyme that catalyzes the conversion of 1.0 μmol substrate to product per min.

RESULTS

Metabolism of 5α-Adiol catalyzed by mitochondrial 17β-HSD10

The K_mA 1 value of NAD⁺ for human mitochondrial 17β-HSD10 catalyzing the conversion of 5α-Adiol to DHT was determined to be 274±51 μM whereas the K_mB¹ value of 5α-Adiol is 21±5 μM. The k_cat¹ = 39.1 x10^–3 ±0.3 x10^–3 s^–1 at pH 7.4. The catalytic efficiency (k_cat/K_mB) of this enzyme in the removal of the neurosteroid 5α-Adiol (see the upper part of Fig. 1) was found to be 112 mM^–1min^–1. This was about three times greater than that of the catalyzing of the DHT reduction by NADH to generate 5α-Adiol at the same pH (7.4) and salt concentration.

Fig. 1

Pre-receptor control of the hormone and neurosteroid metabolism by human mitochondrial 17β-HSD10, a homotetramer shown as four tightly associated yellow balls. Upper part: 5α-Adiol was oxidized by NAD⁺ and the backward reaction displaying the reduction of DHT by NADH; Lower part: The inactivation of 17β- estradiol due to its oxidation by NAD⁺. The mitochondrial matrix is shown in pink.

Effects of pH on the metabolism of 5α-Adiol

The pH profiles of the human 17β-HSD10 activity catalyzing the metabolism of 5α-Adiol were determined. The pH optima for the oxidative conversion of 5α-Adiol to DHT by NAD⁺ was found to be near pH 10 whereas the backward reaction, i.e., the reduction of DHT by NADH to generate 5α-Adiol, gradually decreases as pH declines from 6.6 (Fig. 2). A high pH environment favors the oxidative conversion of 5α-Adiol but not the backward reaction. Since a slightly more basic environment than that of cytoplasm (pH 7.2) is in mitochondrial matrix, the unique localization of 17β-HSD10 in mitochondria increases its efficiency in catalyzing the removal of 5α-Adiol.

Fig. 2

Effects of pH on the human 17β-HSD10 activities in the 5α-Adiol metabolism. Blue and red lines represent 17β-HSD10 activities measured with 100 μM of 5α-Adiol oxidized by 2 mM NAD⁺ and 100 μM of DHT reduced by 100 μM NADH, respectively, at different pH by following the Procedure A as described in the Materials and Methods.

Oxidative inactivation of 17β-estradiol by human 17β-HSD10

Human 17β-HSD10 is unable to use NADP⁺ as a coenzyme to catalyze the dehydrogenation of a substrate (data not shown). The ratio of [NADP⁺]/[NAD⁺] is known to be extremely high in the ER. Since ERAB was reportedly associated with ER [12, 13], it seems impossible for it to play a role in the oxidation of 17β-estradiol despite any putative increase in ERAB’s catalytic efficiency (Table 2). Mitochondrial 17β-HSD10 could effectively catalyze the oxidation of 17β-estradiol (Fig. 3c), a potent estrogen, to estrone, a very weak estrogen, with a catalytic efficiency of 40.9 mM^–1 min^–1, and K_mB value = 22±8.9 μM (Table 2). On the other hand, the catalytic rate of the reduction of estrone by NADH under these experimental conditions is negligible (Fig. 3d). This demonstrates that mitochondrial 17β-HSD10 is a powerful molecular machine for the inactivation of 17β-estradiol (see lower part of Fig. 1).

Fig. 3

The oxidative inactivation of 17β-estradiol and the estrone reduction catalyzed by human 17β-HSD10. The initial velocity (v) was measured by following the Procedure B2 (a curve in b) and A (a curve in c), respectively, as well as by the procedure B1 where the observed absorbance changes with time were shown as a solid circle at every 5 min (a series of solid circles in a). The v values measured were equal to the slope of a tangent at the origin (27), i.e., the ΔA₃₄₀ per min recorded at time near zero. The reduction of estrone catalyzed by 17β-HSD10 was measured by following the Procedure A (d).

Table 2

Oxidative inactivation of 17β-estradiol catalyzed by mitochondrial 17β-HSD10

			K_m
Acronym	V_max(mU/mg)	k_cat (s^–1)	17β-estradiol (μM)	NAD⁺ (μM)	k_cat/K_mB^a (mM^-1 •min^-1)	Assay pH	Ref.
17β-HSD10	32.5±5.8	15x10^–3 ±1.5 x10^–3	22±8.9	255±23	40.9	8.0^b	This study
ERAB/ABAD	23,000±3,000	10±1.3	14±6	ND^c	44,400	8.9	[13]

^aThis is the reported catalytic efficiency. ^bIf the pH were 8.9, the k_cat would increase about 19% according to the published pH profile [7]. ^cNot determined.

Measurement of initial velocity for enzyme kinetics studies on 17β-estradiol oxidation

The initial velocity of the oxidation of 17β-estradiol by NAD⁺ catalyzed by mitochondrial 17β-HSD10 was determined by the experimental procedure B1 and B2 under the same experimental conditions except that the absorbance changes at 340 nm as a function of time was monitored every 5 min for 2 h as described in [13] (Fig. 3a) or a measurement done at the beginning of steady-state (Fig. 3b). A comparison of Fig. 3a to Fig. 3b indicates that the initial velocity obtained by the experimental procedure B1 that followed ‘Experimental Procedures’ previously published in [13] (Supplementary Material 3c) was lower than the actual one shown in Fig. 3b by at least 39%.

DISCUSSION

During the study of 3-hydroxyacyl-CoA dehydrogenase (HAD), a new type of HAD—17β-HSD10—was discovered in human brain [4 , 6–8] (Supplementary Material 3a). This protein was first purified and characterized as a fatty acid oxidation enzyme, namely short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) which is essential for isoleucine metabolism [4, 28]. With the cloning of its cDNA, the identification of the HSD17B10/SCHAD gene and its mapping to Xp11.2 [6], this short-chain 3-hydroxyacyl-CoA dehydrogenase was found to be a new type of 17β-HSD [7, 8] involved not only in the oxidation of fatty acids and branched-chain amino acids [4, 28] but also in the metabolism of neurosteroids [1 , 30], which have large paracrine or autocrine impacts on brain cells. With the cloning and characterization of other mammalian 17β-HSD10s such as the rat and mouse homologs of human 17β-HSD10 [31, 32], it became clear that all the above-mentioned short-chain 3-hydroxyacyl-CoA dehydrogenases including bovine HAD2 [33] belong to a hydroxysteroid dehydrogenase (HSD) family, namely type 10 17β-HSD [2–8 , 31] (Table 1). The enzymatic activity of human 17β-HSD10 is labile to freeze-thaw damage, so in many cases the commercially available 17β-HSD10 preparations lacked activities. Since its enzymatic activities such as 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) and 3α-hydroxysteroid dehydrogenase (3α-HSD) activity were not significantly affected by adding of the N-terminal 6xHis tag [1], we used N-terminal 6xHis tagged HSD10 freshly prepared in the current study.

The product of the oxidation of 5α-Adiol by NAD⁺ by mitochondrial 17β-HSD10 has been identified by TLC to be DHT [29]. On the other hand, 5α-Adiol could be converted to another weak androgen androsterone if an NAD⁺-dependent oxidative reaction was catalyzed by microsomal 17β-HSD6 [34], and quickly removed from the body after glucuronidation and/or sulfation. The 17β-HSD6 is mainly expressed in liver and prostate [34] whereas human 17β-HSD10 is highly expressed not only in prostate but also in hippocampus, hypothalamus and other brain regions [24]. The metabolism of neurosteroids such as allopregnanolone [24, 30], 3α,5α -tetrahydrodeoxycorticosterone [30] and 5α-Adiol [22, 23] in brain requires mitochondrial 17β-HSD10. The critical role of 17β-HSD10 in maintaining the dynamic levels of 5α-Adiol and DHT in brain, prostate, and other important organs [5 , 35–38] needs to be further explored. Its functioning in conditions close to those of the mitochondrial matrix should be carefully studied for the elucidation of the pre-receptor control of neurosteroid metabolism (Fig. 1).

Since mitochondrial matrix is slightly more basic than that in cytosol, the kinetic study on androgen and neurosteroid metabolism should be observed at pH 7.4 where the [H⁺] has shrunk the difference between rates of the forward and backward reaction that is apparent at pH 8.0 (see Fig. 2). Although a higher [H⁺] (pH 5–6) would further inhibit the forward reaction, it would also decelerate the backward reaction which is similar to its impact on the reduction of acetoacetyl-CoA [7] (Fig. 2). A comparison of the enzyme’s forward and backward catalytic efficiencies indicates that at pH 7.4 the conversion of the neurosteroid 5α-Adiol, to the potent androgen DHT, is about three times more favored than the backward reaction. The generation of 5α-Adiol from the reduction of DHT by NADPH is, indeed, catalyzed by the aldo-keto reductase 1C2 (AKR1C2) in the ER [22, 23]. It was reported [39, 40] that [NAD⁺] is about 246 μM in Hela cell mitochondria which is about 6.7-fold higher than [NADH]. It means that probably only half of the mitochondrial 17β-HSD10 has the opportunity to bind NAD⁺ and catalyze the oxidation of 5α-Adiol to DHT assuming that this enzyme is near the mitochondrial inner membrane where the respiratory electron transfer chain continuously generates NAD⁺ from NADH. Since 17β-HSD10 and its coenzymes are confined by the double membrane system, mitochondria appear to be a unique organelle in the pre-receptor control of neurosteroid metabolism (Fig. 1).

Since mitochondrial 17β-HSD10 is a multifunctional protein catalyzing several metabolic pathways simultaneously [2, 4], different substrates would compete for access to the active site of 17β-HSD10 dependent upon their individual K_mB. In other words, substrates other than 5α-Adiol and DHT in the mitochondrial matrix might act as competitive ‘inhibitors’ to the removal of 5α-Adiol. Thus, the potential activity of mitochondrial 17β-HSD10 should be smaller than that estimated by considering only the K_mA of NAD⁺ and [NAD⁺] in mitochondrial matrix. With respect to the ‘one-site competitive inhibition model’ reported in [13] (see Supplementary Material 3c), it was found that there is no such a model mentioned in the cited reference (see reference 34 in [13]) which is a standard reference for enzyme kinetics [27].

Although the enzyme concentration up to 150 μg/ml used in this study is relatively high, [E] was still< < [S] (61 μM) so that [ES] would rapidly reach a steady-state under these conditions. Since the intracellular concentration of K⁺ is about fourteen times higher than that of Na⁺, potassium phosphate buffer (pH 8.0), is more appropriate than in sodium phosphate buffer (pH 8.9) (used in [13]) for the determination of enzyme activities of ERAB/ABAD’s oxidative inactivation of 17 β-estradiol. The kinetic parameters of this dehydrogenase for the catalysis of 17β-estradiol oxidation by NAD⁺ were found to be k_cat = 15 x10^–3±1.5 x10^–3 s^–1 and K_mB = 22±8.9 μM. Its catalytic efficiency k_cat/K_mB = 40.9 mM^–1min^–1 is close to the previously reported data [7] (Table 2), but is drastically less than that in a frequently cited publication [13]. If the kinetic data in Table 1 of [13] (see Supplementary Material 3b) were accurate, the substrate (17β-estradiol from 3.8 to 92 μM) would be virtually depleted within a few seconds—twenty seconds at most—in the presence of the enzyme at 30 μg/ml [13]. Under these circumstances, stopped-flow measurements of NADH formation with a scale of 100 ms/small division [41] would be the appropriate technique, rather than the monitoring of absorbance at 340 nm every 5 min for 2 h as reported in [13] (see Supplementary Material 3c). 17β-HSD10 uses coenzyme I rather than II as the coenzyme [4], which is bound to this enzyme first and dissociates from the 17β-HSD10 • NADH complex after the product has left the active site. It is essential to determine the initial velocities (v) at different concentrations of substrate and coenzyme to study steady-state kinetics of such an ordered bireactant system [27]. The initial velocity is, by definition, the rate during the early stage of the enzymatic reaction [27]. For this determination, it makes no sense to run the reaction for 2 h and monitor absorbance at 340 nm every 5 min for 2 h as reported in [13] (Fig. 3a). There are also additional questions: As shown in Fig. 3a, the “initial velocity” obtained by following the Experimental Procedures of [13] (Supplementary Material 3c) and Fig. 2 legend of [13] (Supplementary Material 2d), was less than the actual one by at least 39% (Fig. 3b). However, the catalytic rate of the enzyme (k_cat) toward 17β-estradiol reported in [13] was about three orders of magnitude greater than what could be obtained from those experiments as described in [13] (Table 2). Data about the enzymatic activities with other substrates published in the Table 1 of [13] (Supplementary Material 3b) were also exaggerated to a different extent. For example, the reduction rate of acetoacetyl-CoA by NADH under the catalysis of 17β-HSD10/ERAB was the one relatively close to the true value [6], exaggerated by only 5 times to k_cat = 190 s^–1 [13]; note that the k_cat = 37 s^–1 had already been published in JBC and in the NIH news coverage (Supplementary Material 3a). It is most likely that the data summarized in Table 1 of [13] (Supplementary Material 3b) were not based upon those described experiments. Although it seems unlikely that such erroneous data could be due to a general computational mistake, neither the authors nor the journals have offered a corrigendum.

The magic term ABAD [14], Aβ binding alcohol dehydrogenase, was first proposed at the end of [13] based upon its ‘generalized alcohol dehydrogenase activity’ shown in Table 1 of [13] (Supplementary Material 3b) and Fig. 2 of that report (Supplementary Material 2c). As is well known, (–)-2-octanol, (+)-2-octanol, and (±)-2-octanol are oils at 25°C [42]. The solubility of (–)-2-octanol and (+)-2-octanol are only 6 mM and 8.5 mM, respectively. Since it is impossible for the solubility of racemic 2-octanol to be greater than 15 mM, it is unclear how a high concentration of (–)-2-octanol solution (160 mM to 210 mM) can be prepared without solubilization treatments as published previously (see Fig. 2 of [13] or Supplementary Material 2d, Fig. 5 of [13] or Supplementary Material 2e as well as Table 1 of [13] or Supplementary Material 3b and 3c). However, the K_m values for (–)-2-octanol, (+)-2-decanol, and racemic (±)-2-octanol were reported to be 43, 84, and 85 mM, respectively [13] (Supplementary Material 3b). More surprisingly, the reported V_max value of ERAB/ABAD for catalyzing the oxidation of racemic 2-octanol was much greater than a sum of the rates for each enantiomer (Supplementary Material 3b). If the successful preparation of high concentration 2-octanol or n-decanol solution appears to be unlikely under the reported experimental conditions (Supplementary Material 3c), related journals should seriously face the problem as those problematic publications have been repeatedly cited even to the present time [15 –17].

The results show that this dehydrogenase is unable to effectively catalyze the reduction of estrone by NADH (Fig. 3d). Therefore, human 17β-HSD10 plays a role in the metabolism of estrogenic hormone by ‘unidirection’ catalysis, namely the oxidative inactivation of 17β-estradiol (Table 2). The molecular mechanism of ‘unidirection’ catalysis remains to be elucidated 2 . It is generally believed that 17β-estradiol, a potent estrogen, has protective effects on the nervous system [43, 44]. For example, 17β-estradiol could reduce the AD risk by lowering the generation of Aβ due to the alteration of the metabolism of AβPP and its trafficking [45].

17β-estradiol is a neuroactive steroid which can be locally generated by brain cells in addition to its accumulation from the bloodstream. Its brain levels are also determined by its inactivation speed in brain. Results of the current study demonstrate that some of 17β-estradiol molecules are inactivated in mitochondria by 17β-HSD10 (Fig. 1). It was reported [46] that when 17β-HSD10 bound to Aβ or its fragments, its enzymatic activity is inhibited. Since 17β-estradiol would inhibit the binding of 17β-HSD10 to estrogen receptor alpha in mitochondria [9], the involvement of 17β-HSD10 in the pre-receptor control of estrogen metabolism is rather complicated. Its real impact on AD is certainly more complex than previously recognized [47]. It was reported [12 , 30] that significantly increased expression of 17β-HSD10 in brain cells is a prominent feature in AD patients. Results of the current study suggest that overexpression of 17β-HSD10 would alter neurosteroid levels in different brain regions. This may disturb the balance between pro-inflammatory and anti-inflammatory cytokines, increasing neuroinflammation and lead to the deterioration of cognitive function [48]. With the correction of ERAB/ABAD episode, roles of mitochondrial 17β-HSD10 in neurodegeneration deserve to be further studied for the elucidation of AD pathogenesis.

AKNOWLEDGMENTS

We are in debt to Drs. D. Miller and C. Dobkin for invaluable advice. This work was supported in part by the New York State Office for People With Developmental Disabilities.

Authors’ disclosures available online (https://www.j-alz.com/manuscript-disclosures/17-0974r2).

Footnotes

The steady-state kinetics of 17β-HSD10, an allosteric NAD⁺-dependent dehydrogenase [1, 6], was known to be an Ordered Bi-Bi system []: A+E ⇆ EA+ B ⇆ EAB ⇆ EPQ ⇆ P+EQ ⇆ E+Q. Here A is NAD⁺ while Q represents NADH. B is the substrate and P the product. K_mA is the Michaelis constant of NAD⁺ and K_mB is that of individual substrates. The catalytic rate constant, k_cat, is the maximal number of substrate molecules converted to product per second (s^-1) in the active site.

Theoretically, the enzyme catalysis would accelerate the reaction velocity in both directions.

The supplementary material is available in the electronic version of this article: .

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