Studies on triglyceride metabolism: phosphatidate phosphohydrolase from guinea pig harderian gland

The guinea pig Harderian gland, located in the orbit, is characterized by a high production of lipids. However, little is known about the regulation of the metabolic pathways involved. In the present paper the properties of guinea pig Harderian gland phosphatidate phosphohydrolase, a key enzyme in triglyceride biosynthesis, was investigated. The enzyme was present both in the cytosolic and the microsomal fraction from the gland. Cytosolic phosphatidate phosphohydrolase was purified by hydroxylapatite chromatography. The enzyme was dependent on magnesium ions and inhibited in the presence of fluoride. The dependence on the substrate phosphatidate was investigated and the apparent Km for phosphatidate was about 0.6 mM. Phosphatidate phosphohydrolase activity was influenced by different phospholipids. As is the case for the rat liver enzyme, phosphatidylethanolamine was found to stimulate the enzyme activity. The results indicate that Harderian gland phosphatidate phosphohydrolase has similar properties as the corresponding enzyme from rat liver, suggesting that it may be of regulatory importance.