Effect of valine on propionate metabolism in control and hyperglycinemic fibroblasts and in rat liver

Abstract

Summary: Measurement of methylmalonyl-CoA mutase and propionyl- CoA carboxylase activities in lysates from fibroblasts derived from control, nonketotic hyperglycinemia, propionic acidemia, and both vitamin B12-responsive and -nonresponsive variants of methylmalonic acidemia showed only one abnormality: a 59% decrease in carboxylase activity in the nonketotic hyperglycinemic lysates (P < 0.01). When fibroblasts from all cell types were grown on valine-supplemented (24 mM) media, mutase activity was generally inhibited. As for carboxylase activity, control lines were inhibited 35% as compared to controls without valine and propionic acidemia activity was undetectable. On the other hand, carboxylase activity in both methylmalonic acidemia variants was increased 40% and nonketotic hyperglycinemia carboxylase activity was increased 80% (P < 0.01) when grown on valine-supplemented media. Isoleucine could not substitute for valine in producing increased carboxylase activity in these mutants. Glycine cleavage activity in fresh rat liver homogenates (11.1 µmol/mg protein/90 min) did not vary significantly when 24 mM valine was added to the reaction (9.9 µmol/mg protein/90 min). Therefore, the hyperglycinemia observed in both ketotic and nonketotic forms is probably not caused by a direct effect of valine on the glycine cleavage reaction. These data suggest that the presence of increased amounts of propionic acid in scrum or urine does not necessarily rule out the possibility of nonketotic hyperglycinemia due to the decreased activity of the carboxylase enzyme. Speculation: Decreased activity of the propionyl-CoA carboxylase enzyme in nonketotic hyperglycinemia fibroblasts suggests altered propionate metabolism in this mutation. Additionally, growth of nonketotic hyperglycinemia fibroblasts on high concentrations of valine results in greatly increased carboxylase activity. This effect of valine is also evident in methylmalonic acidemia lysates although no decrease in carboxylase activity in these mutants grown under normal conditions is observed. Although the valine effect is not presently understood, it may be possible that nonketotic hyperglycinemia can be diagnosed from fibroblasts grown in tissue culture based on the combination of these two findings: (1) decreased propionyl-CoA carboxylase activity in nonketotic hyperglycinemic lysates grown under normal culture conditions and (2) increased propionyl-CoA carboxylase activity in nonketotic hyperglycinemic lysates grown on valine-supplemented media. © 1977 International Pediatric Research Foundation, Inc.