The mechanism of alkyldihydroxyacetone-P synthase. Formation of [3H]H2O from acyl[1-R-3H]dihydroxyacetone-P by purified alkyldihydroxyacetone-P synthase in the absence of acylhydrolase activity.

Alkyldihydroxyacetone-P (alkyl-DHAP) synthase catalyzes the exchange of the fatty acid esterified to C-1 of the DHAP portion of acyl-DHAP for a fatty alcohol to form 1-O-alkyl-DHAP, the first ether-linked intermediate in ether lipid biosynthesis. Another characteristic of the reaction is the exchange of the pro-R hydrogen at C-1. We have investigated this hydrogen exchange using palmitoyl-[1-R-3H]DHAP and a 1000-fold purified preparation of alkyl-DHAP synthase. We found a small but significant pro-R hydrogen exchange in the absence of the co-substrate, fatty alcohol. When [14C]hexadecanol was added, the increase in pro-R 3H exchange was equal to the [14C]hexadecyl-DHAP formed. Addition of [14C]palmitic acid resulted in an increase in pro-R 3H exchange that matched the formation of [14C]palmitoyl-DHAP by the acyl exchange activity of alkyl-DHAP synthase. Furthermore, although whole microsomes contain at least two acyl hydrolases for acyl-DHAP, purified preparations of alkyl-DHAP synthase do not form DHAP from acyl-DHAP. These results are discussed with respect to data obtained from other laboratories using whole microsomes and in support of our proposed ping-pong mechanism for alkyl-DHAP synthase.