Dietary effects on the formation of dolichyl monophosphate mannose by microsomal preparations of rat adipose tissue

Microsomal preparations from rat adipose tissue catalyse the transfer of ¹⁴C]mannose from GDP-¹⁴C]mannose to an endogenous acceptor forming a ¹⁴C]mannosyl lipid. The mannosyl lipid co-chromatographs with hen oviduct dolichyl monophosphate β-mannose on three solvent systems. It is stable to mild alkaline hydrolysis, but strong alkaline treatment yields a compound that co-migrates with mannose 1-phosphate. The mannosyl lipid is labile to mild acid hydrolysis, yielding ¹⁴C]mannose. Formation of the compound is reversible by GDP, but not GMP, and is stimulated by exogenous dolichyl phosphate.

The kinetics of transfer of ¹⁴C]mannose from GDP-¹⁴C]mannose to form dolichyl monophosphate mannose were studied by using preparations derived from rats fed on one of four diets: G (high glucose), L (high lard), F (fructose) or GC (high glucose, 0.9% cholesterol). The K_m and V_max. values for transfer from GDP-mannose were virtually indistinguishable in the four preparations.

In the absence of exogenous dolichyl phosphate, the largest amount of transfer of ¹⁴C]mannose into the mannosyl lipid was observed with preparations from fructose-fed animals. Preparations from glucose-fed animals showed about 60% as much transfer, whereas membranes from rats fed the other diets showed intermediate values between the fructose- and glucose-fed animals. The inclusion of cholesterol in the glucose diet elicited an increase in transfer of mannose.

Under conditions of saturating exogenous dolichyl phosphate, preparations from lard-fed animals have 1.5 times as much enzyme activity as do preparations from animals fed the other three diets.