Effect of an exogenous succinyl-CoA-generating system on the measurement of delta-aminolevulinic acid synthase activity in rat liver tissue by a radiochemical assay.

Rat liver tissue was used to examine the effect of an exogenous succinyl-CoA-generating system on the radiochemical assay for delta-aminolevulinic acid synthase (succinyl-CoA:glycine C-succinyltransferase (decarboxylating), EC 2.3.1.37) activity developed by Ebert et al. (Ebert, P.S., Tschudy, D.P., Choudry, J.N. and Chirigos, M.A. (1970) Biochim. Biophys. Acta 208, 236--250). In the absence of exogenous succinate thiokinase, 34--62% (average 55%) of the radioactivity in the final column eluate could be attributed to delta-amino-[4-14C]levulinic acid, as assessed by conversion of delta-aminolevulinic acid in the eluate to a pyrrole. The addition of succinate thiokinase markedly enhanced the formation of the contaminant(s), as succinyl-CoA was metabolized to a compound or compounds that eluted chromatographically with delta-amino-levulinic acid. This effect was abolished by 10 mM EDTA, probably because the generation of succinyl-CoA was suppressed due to the chelation of Mg2+. These observations indicate that this radiochemical assay should be carefully examined for each set of assay conditions employed.