SummaryWe report some properties of propionyl CoA carboxylase (PCC) partially purified from cultured human fibroblasts obtained from controls and several patients with propionic acidemia. A series of steps (Triton X-100 treatment, high speed centrifugation, ammonium sulfate precipitation, and density gradient centrifugation) led to 100− to 300− fold purification of control enzyme. Control PCC had a molecular weight of nearly 700,000, contained biotin, demonstrated a pH optimum at 8.0–8.5, was activated by potassium, and followed Michaelis-Menten kinetics for each of its substrates. It was distinguished from acetyl CoA carboxylase immunologically as well as by differential purification.Each of seven lines from patients with propionic acidemia had clearly detectable PCC activity which was less than 5% of that in control lines. Although yields were poor and purification less extensive than in control lines, mutant PCC was enriched 2− to 40-fold by the same procedures employed for the control enzyme. Mutant enzyme had a pH optimum, ionic requirements, and substrate Km's similar to those of control PCC, but was distinctly more labile to both cold and heat. These findings suggest that the markedly reduced activity of PCC in these patients results from a mutation in the PCC structural gene locus or loci which leads to the synthesis of altered enzyme protein molecules.SpeculationOur results suggest that human propionyl CoA carboxylase has properties very similar to those of other mammalian propionyl CoA carboxylases. Further chemical studies with control enzyme and that from patients with propionic acidemia may help define the basis for the two major genetic complementation groups reported among propionyl CoA carboxylase deficient patients.