Characterization of the Reaction of GDP-Mannose with Dolichol Phosphate in Liver Membranes

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Abstract
The Mn2+-dependent mannosyl transfer reaction between GDP-[14C]mannose and dolichol phosphate, which is catalyzed by liver membranes, could not be followed accurately with the existing assay systems. Thus, GDP-[14C]mannose is hydrolyzed rapidly by a pyrophosphatase present in microsomal and Golgi fractions from liver cells. The rate of the hydrolysis is rapid enough to limit the extent of incorporation of [14C]mannose into endogenous acceptors. AMP was an effective inhibitor of the pyrophosphatase in Golgi membranes, and protected GDP-mannose from metabolism in alternative pathways. In the presence of AMP it was possible accurately to follow the time course of synthesis of dolichol phosphate [14C]mannose over short time periods. Even though the time course of the reaction was measured over 2 s intervals, no linear portion could be detected in plots of product formed versus time. The kinetics of synthesis did, however, fit an equation for a first-order kinetic process. The basis for the first-order kinetics seems related to the very small amounts of dolichol phosphate in membranes. The values of the first-order rate constant is dependent on the concentrations of GDP-mannose and Mn2+ added to the assays.