Rabbit Muscle Triosephosphate Isomerase: Activity and Inactivation by Sulfhydryl Reagents as Affected by Enzyme Concentration 1

Summary

Progressive increase of enzymatic activity with time occurs after dilution of a crystalline suspension of rabbit muscle TIM.

The concentration of p-MB required to inhibit the enzyme is higher the greater the concentration of enzyme. With the lowest concentrations of mercurial that are ultimately inhibitory to the enzyme in dilute solution, there is a lag period before inactivation begins. With higher concentrations of mercurial, some degree of inactivation occurs almost immediately, and enzymatic activity thereafter decreases at a slower rate. With an enzyme concentration of 32 ng/ml, a low concentration of p-MB decreases V _max without change of K_M . A higher concentration of mercurial also increases K_M .

With an enzyme concentration of 1 mg/ml, iodoacetate begins to inactivate it only after a lag period. In enzyme solutions containing 8 ng/ml, inactivation by iodoacetate progresses without significant initial lag.

Discrepancies between the present work and that of other authors can probably be accounted for by differences in the concentrations of enzyme used. It is suggested that the conformation of rabbit muscle TIM in solution is dependent on its concentration. In more dilute solutions the enzyme assumes a conformation in which it is catalyt-ically more active and in which its sulfhydryl groups are more accessible to chemical attack.