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

Abstract

Conditions required for optimal assay of low levels of activity of hepatic δ-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 δ-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 δ-aminolevulinic acid synthetase under these conditions. Of the metabolic inhibitors tested (F −, malonate, and arsenite), only F − (100 m m final concentration) enhanced activity. Increasing the permeability of mitochondria by quick freezethawing of fresh homogenates just before assay did not increase the rate of δ-aminolevulinic acid formation.