Mannosyl Retinyl Phosphate: Its Role as a Donor of Mannose to Glycoconjugates in Rat Liver Membranes

Abstract

The chapter describes the role of vitamin A (retinol) in the visual cycle. Phosphate and glycosyl phosphate derivatives of retinol are synthesized in vitro and in the whole animal. These compounds are labile to yield anhydroretinol, which is a compound with a characteristic absorption spectrum with maxima at 350, 370, and 390 nm. Monophosphate and glycosyl monophosphate derivatives of dolichol are stable under such conditions. In vivo , rat liver cells synthesize mannosyl retinylphosphate (MRP) and dolichyl mannosyl phosphate (DMP) in the ratio of approximately 1:1, whereas in vitro mostly DMP is obtained from guanosine diphosphate mannose. Addition of retinyl phosphate (RP) to the liver microsomalsystem specifically enhances MRP synthesis. MRP transfers mannosyl residues to endogenous glycoconjugates of rat liver microsomes.