The glyoxylate cycle in rat epiphyseal cartilage: the effect of vitamin-D3 on the activity of the enzymes isocitrate lyase and malate synthase.

The effect of vitamin-D deficiency and subsequent vitamin-D replacement on the metabolism of rat epiphyseal growth plate cartilage was studied. Biochemical analyses showed the presence of the two unique glyoxylate cycle enzymes isocitrate lyase and malate synthase in cartilage. The activity of these enzymes was markedly increased after treatment with the vitamin. Additionally, rat cartilage showed the capacity to oxidize fatty acid in the presence of cyanide. This cyanide-insensitive fatty acid oxidation is characteristic of peroxisomal B-oxidation rather than mitochondrial B-oxidation. Vitamin-D treatment also increased fatty acid oxidation. Lastly, incubation of rat cartilage in the presence of a fatty acid substrate such as palmitate, resulted in a higher tissue glycogen content. Tissue glycogen was further elevated by vitamin-D. Such data indicate the presence of glyoxylate cycle enzymes in a vertebrate tissue and raise the possibility that mammalian cartilage has the capacity to convert lipid to carbohydrate.