Reductase, Phosphatase, and Kinase Activities in the Metabolism of Alkyldihydroxyacetone Phosphate and Alkyldihydroxyacetone

Abstract

Abstract Kinetic studies have been carried out with microsomal preparations from mouse preputial gland tumors that catalyze (a) the reduction of [1-14C]alkyldihydroxyacetone-P and [1-14C]alkyldihydroxyacetone; (b) the dephosphorylation of [1-14C]alkyldihydroxyacetone-P; and (c) the phosphorylation of [1-14C]alkyldihydroxyacetone. Our data demonstrate the existence of a pathway that permits the interconversion of metabolic intermediary ether-lipid products, i.e. alkyldihydroxyacetone-P, alkylglycerol-P, alkyldihydroxyacetone, and alkylglycerol. The labeled substrates used in these experiments, [1-14C]octadecyldihydroxyacetone-P and [1-14C]-octadecyldihydroxyacetone, were chemically synthesized. Alkyldihydroxyacetone-P is reduced to 1-alkyl-sn-glycerol-3-P by a microsomal reductase that is highly specific for NADPH and optimally active at pH 7.0. However, higher levels of NADH can substitute for the NADPH. The apparent Km for the alkyldihydroxyacetone-P reductase is 0.12 mm for NADPH and 3.2 mm for NADH. In contrast, only NADPH is utilized by the reductase that reduces alkyldihydroxyacetone to the alkylglycerol. With the alkyldihydroxyacetone reductase, the apparent Km for NADPH is 0.38 mm. The phosphatase present in the tumor microsomes is maximally inhibited at NaF concentrations greater than 40 mm; less than 4% of the alkyldihydroxyacetone-P was dephosphorylated during a 5-min incubation under these conditions. However, during longer incubation periods at low concentrations of NaF, significant quantities of alkyldihydroxyacetone and alkylglycerol are produced. In the presence of ATP and Mg2+, alkyldihydroxyacetone is phosphorylated to alkyldihydroxyacetone-P by a kinase in the tumor. The apparent Km for ATP was 3.6 mm. The highest specific activity for the alkyldihydroxyacetone kinase was located in the microsomal fraction. These data demonstrate how alkyldihydroxyacetone can re-enter the pathway responsible for the biosynthesis of complex ether lipids.