Assay of delta-aminolevulinic acid synthetase in homogenates of mouse, rat, and human liver: species differences in requirement for an exogenous succinyl-CoA-generating system.

Conditions required for optimal assay of low levels of activity of hepatic delta-aminolevulinic acid synthetase have been studied, comparing dilute homogenates of mouse, rat, and human livers. The assay method used was a modification of that described by Ebert et al. (Biochim, Biophys, Acta (1970) 208, 236-250), and livers were studied from both untreated animal and human subjects and subjects pretreated with porphyrinogenic compounds. In homogenates of mouse and human but not rat liver, maximal rates of delta-aminolevulinic acid formation required addition to the incubation mixture of an exogenous system for succinyl-CoA generation. The requirement for this generating system was increased if livers from pretreated subjects were frozen and stored prior to assay, suggesting that the endogenous capacity for succinyl-CoA generation was more labile than delta-aminolevulinic acid synthetase under these conditions. Of the metabolic inhibitors tested (F-, malonate, and arsenite), only F- (100 mM final concentration) enhanced activity. Increasing the permeability of mitochondria by quick freeze-thawing of fresh homogenates just before assay did not increase the rate of delta-aminolevulinic acid formation.