PRECURSOR ROLE OF BRANCHED-CHAIN AMINO-ACIDS IN THE BIOSYNTHESIS OF ISO AND ANTEISO FATTY-ACIDS IN RAT SKIN

Monoester fraction of rat skin surface lipid has been shown to contain more than trace amounts of branched-long-chain fatty acids (BCFAs) of the iso and anteiso series. These BCFAs are biosynthesized using eith er branched-chain amino acids (BCAAs) or branched-chain cr-keto acids (BCKAs), or using both of them as precursor. This study has been carri ed out to address which precursor, BCAAs or BCKAs in the circulation, are mainly utilized for biosynthesis of BCFAs. Dietary supplement of [ C-14]-valine and isoleucine-induced sharp rise of serum concentration of these two amino acids and their respective cr-keto acids, and eleva ted the levels of related BCFAs and branched-chain fatty alcohols in t he monoester fraction. A larger proportion of label in the total skin surface lipid was found in the monoester fraction in which fatty acid and alcohol accounted for approx. 80% of total radioactivity. Incorpor ation of intravenously administered [C-14]-BCAAs and BCKAs into the mo noester fraction revealed that BCAAs were far better as precursors tha n BCKAs for BCFA biosynthesis in rat skin. Among three BCAAs, leucine differed from valine or isoleucine in that this amino acid was primari ly utilized for production of straight-chain fatty acids rather than f or production of related BCFA.