Early Labeled Heme Synthesis from Delta-Aminolevulinic Acid-4-[ 14 C] in Rats: Comparison with Glycine-2-[ 14 C] 1

Summary

The early phases of heme synthesis were measured with ALA-[¹⁴C] as precursor. Two early peaks of heme labeling were observed in the liver, as found in earlier studies with glycine-2-[¹⁴C]. However, hepatic heme labeling with ALA differed from that observed with glycine in several respects. There was much poorer discrimination of the two early heme peaks, with more rapid formation of the second component. Formation of both peaks occurred more rapidly in microsomes and mitochondria as compared to whole liver, and there was a larger contribution of the microsomal fraction to total hepatic heme labeling. Also in contrast to the findings with glycine-[¹⁴C], there was no distinct early heme peak in the peripheral blood. These differences may be due to the fact that ALA enters the pathway of heme biosynthesis beyond the primary rate-limiting step and could have selective access to certain sites of heme formation. ALA is often employed as a selective label for liver hemes. However, glycine may be required for more definitive assessment of the kinetics and magnitude of labeled heme synthesis.