On the formation of alpha-hydroxy fatty acids. Evidence for a direct hydroxylation of nonhydroxy fatty acid-containing sphingolipids.

Abstract

When Tetrahymena pyriformis, strain NT-1, was warmed from a growth temperature of 15 degrees C to 39 degrees C, there was a rapid conversion of its sphingolipids from the nonhydroxy fatty acid-containing ceramide aminoethylphosphonate (NCAEP) predominant at low temperature to the alpha-hydroxy fatty acid-containing analog (HCAEP). The fatty acid composition of the freshly formed HCAEP strongly resembled that of the original NCAEP pool. An extensive utilization of NCAEP long chain bases for HCAEP formation was shown by prelabeling the NCAEP bases with [14C]serine. The specific radioactivities of both fatty acids and long chain bases of the two sphingolipid classes of cells prelabeled with [3H]palmitic acid at 15 degrees C and then warmed to 39 degrees C were compatible with a direct hydroxylation of the intact ceramide aminoethylphosphonate or free ceramide formed from it. Exogenously added alpha-hydroxypalmitic acid was not incorporated into the sphingolipids intact, but there was an active alpha-oxidation of alpha-hydroxy fatty acids, yielding nonhydroxy fatty acids one carbon atom shorter in length. In vitro experiments failed to demonstrate a direct hydroxylation of NCAEP, and nonhydroxy fatty acid-containing free ceramides were hydroxylated only sparingly. This inefficient hydroxylation is attributed to the rapid enzymatic hydrolysis of these substrates in vitro and to a limited availability of the added ceramides to the hydroxylating enzymes. The weight of the evidence from this study strongly favors the alpha-hydroxylation of fatty acids only when they are bound as elements of sphingolipids.