Metabolism of Glycolate and Glyoxylate in Intact Spinach Leaf Peroxisomes

Abstract

Intact and broken (osmotically disrupted) spinach (Spinacia oleracea) leaf peroxisomes were compared for their enzymic activities on various metabolites in 0.25 molar sucrose solution. Both intact and broken peroxisomes had similar glycolate-dependent o(2) uptake activity. In the conversion of glycolate to glycine in the presence of serine, intact peroxisomes had twice the activity of broken peroxisomes at low glycolate concentrations, and this difference was largely eliminated at saturating glycolate concentrations. However, when glutamate was used instead of serine as the amino group donor, broken peroxisomes had slightly higher activity than intact peroxisomes. In the conversion of glyoxylate to glycine in the presence of serine, intact peroxisomes had only about 50% of the activity of broken peroxisomes at low glyoxylate concentrations, and this difference was largely overcome at saturating glyoxylate concentrations. In the transamination between alanine and hydroxypyruvate, intact peroxisomes had an activity only slightly lower than that of broken peroxisomes. In the oxidation of NADH in the presence of hydroxypyruvate, intact peroxisomes were largely devoid of activity. These results suggest that the peroxisomal membrane does not impose an entry barrier to glycolate, serine, and O(2) for matrix enzyme activity; such a barrier does exist to glutamate, alanine, hydroxypyruvate, glyoxylate, and NADH. Furthermore, in intact peroxisomes, glyoxylate generated by glycolate oxidase is channeled directly to glyoxylate aminotransferase for a more efficient glycolate-glycine conversion. In related studies, application of in vitro osmotic stress to intact or broken peroxisomes had little effect on their ability to metabolize glycolate to glycine.