Glycerol Kinase and Dihydroxyacetone Kinase in Rat Brain

Abstract —The enzymatic phosphorylation of glycerol and dihydroxyacetone by ATP to sn ‐glycerol‐3‐phosphate and dihydroxyacetone phosphate respectively in various subcellular fractions of rat brain was studied. A sensitive radiochemical assay was used where the labelled phosphorylated products were separated from the radioactive substrates by high voltage paper electrophoresis and the radioactivity in these compounds determined. Using this assay the glycerol kinase (EC 2.7.1.30) activity was found to be associated with the mitochondrial fraction of the brain. Under optimum conditions 2.45 nmol of glycerol was phosphorylated/min per mg of protein. The K m for glycerol was 70 μ m at pH 7. This mitochondrial enzyme, like other glycerol kinases from different sources, also phosphorylated dihydroxyacetone. Under optimum conditions 1.7 nmol of dihydroxyacetone phosphate was formed/min per mg of mitochondrial protein. The K m for dihydroxyacetone was 0.6 m m . Glycerol kinase activity was also present in the cytoplasm of brain. However, the specific activity of this enzyme in cytosol is about 15% of the mitochondrial glycerol kinase. Compared to glycerol, dihydroxyacetone was phosphorylated by ATP in cytoplasm at a much higher rate. The pH optimum for this soluble dihydroxyacetone kinase was much lower (pH 6.5) than that of the soluble or mitochondrial glycerol kinase (pH 10.0). Using ammonium sulfate, brain cytoplasm was fractionated to yield a fraction in which the dihydroxyacetone kinase was enriched 2–3 fold with no glycerol kinase activity. Under optimum conditions 1.0 nmol of dihydroxyacetone was phosphorylated/min per mg protein. The K m for dihydroxyacetone was 60 μ m . This cytosol fraction was also found to phosphorylate d ‐glyceraldehyde and l ‐glyceraldehyde at a rate of 30–40% to that of the dihydroxyacetone phosphorylation. The properties and the possible metabolic role of these enzymes in brain are discussed.