Characterization of a retinylmonophosphatase in the plasma membrane of mouse brain.

Retinylmonophosphatase (RMPase) activity in mouse brain paralleled the subcellular distribution of the plasma-membrane marker Na+ + K+-dependent ATPase. The enzyme had a pH optimum between 5.5 and 7.0. The enzyme demonstrated linear kinetics with respect to time and both protein and substrate concentrations. RMPase was saturated by low retinyl monophosphate (RMP) concentrations and exhibited an apparent Km of 4.6 microM. The enzyme did not require MgCl2 for activity, and in fact assays were routinely run in the presence of 10 mM-Na2EDTA. In general, detergents inhibited the enzyme, with 0.05% Triton X-100 causing a 30% loss of activity. Phosphatidic acid was also inhibitory, but phosphatidylcholine and sphingomyelin stimulated phosphatase activity. RMPase was inhibited 35% by 5 mM concentrations of fluoride, phosphate or pyrophosphate. A series of other phosphorylated compounds, including glucose 6-phosphate, alpha-glycerophosphate, ATP, AMP, p-nitrophenyl phosphate and thiamin pyrophosphate, showed little or no inhibition. RMPase activity differed in several characteristics from that previously reported for dolichylmonophosphatase. It is concluded that RMP could play a distinct role in the plasma membrane.