alpha-Hydroxylation of fatty acids in brain. Substrate specificity and deuterium isotope effect

The alpha-hydroxylation of fatty acids with various chain length by a rat brain postnuclear preparation was investigated. The maximum velocity of the alpha-hydroxylation were higher for lignoceric (tetracosanoic), tricosanoic, docosanoic, and heneicosanoic acids and lower for hexacosanoic, pentacosanoic, eicosanoic, and nonadecanoic acids. Stearic palmitic, and phytanic (3,7,11,15-tetramethylpalmitic) acids were not hydroxylated by this preparation. Apparent Km values were lower for fatty acids with the higher Vmax and higher for those with the lower Vmax values. The relative order of alpha-hydroxylation strongly resembles the relative distribution of these fatty acids in brain cerebrosides, and we suggest that alpha-hydroxylation plays an important role in determining this distribution. An improved synthesis of [U-14C]phytanic acid is described. The rate of alpha-hydroxylation with [2-2H2]lignoceric acid was one-fifth of nondeuterated lignoceric acid and one-third of [3-2H2]lignoceric acid, while apparent Km values for these acids were similar. The reduced rate with [2-2H2]lignoceric acid indicates that the cleavage of the C-H bond is the rate-limiting step of the alpha-hydroxylation.