Aliphatic acylcarnitines isolated from a water-soluble fraction of beef heart have been characterized by gas chromatography and mass spectrometry. The following acyl residues derived from the acylcarnitine fraction were unequivocally identified: acetyl, pro ionyl, isobutyryl, butyryl, a-methylbutyryl, valeryl, isovalery, tiglyl, and caproyl. flmethylcrotonyl and methacrylyl were tentatively identified. This occurrence of considerable quantities of branched-chain acylcarnitines indicates a role for carnitine in branched-chain amino acid metabolism.Although it is well established that carnitine participates in shuttling long-chain acyl residues across the inner membrane of mitochondria (1-4), some data indicate additional functions for carnitine. Other roles for carnitine are indicated by (i) the multiorganelle location of medium-chain and short-chain carnitine acyltransferases in liver (5-7), (ii) the high activity of carnitine octanoyltransferase in many tissues (8-10), (iii) the occurrence of short-chain acylcarnitines, especially in muscle, some of which apparently are not acetylcarnitine (11-15), and (iv) the clofibrate-induced increase in short-chain and medium-chain carnitine acyltransferase activity (16)(17)(18)(19)).An approach to elucidating broader roles for carnitine would be to identify and quantitate the products of the short chain and medium chain carnitine acyltransferases in specific tissues. These products should be water-soluble and would be found in the water-soluble acylcarnitine fraction. Herein we show that aliphatic branched-chain acylcarnitines constitute a major portion of the water-soluble carnitine fraction isolated from beef heart. METHODS Isolation of Acylcarnitines. Fresh beef heart was obtained from the slaughterhouse and immediately chilled. The fat was removed and the acylcarnitine fractions were isolated therefrom. Water-soluble aliphatic acylcarnitines were isolated and the acyl moieties were separated as described elsewhere (20).Identification of Acyl Residues. The short-chain fatty acids derived from the acylcarnitine fraction were subjected to gas chromatography as described (20) for tentative identification. Positive identification was made by subjecting the individual fatty acid peaks obtained from gas chromatography to mass spectral analyses and comparing with standard free fatty acids. These analyses were performed by the mass spectrometry facilities at Michigan State University, under the supervision of C. C. Sweeley.
RESULTSThe water-soluble acylcarnitine fraction was isolated from several rat tissues and from sheep muscle and beef heart. As anticipated, considerable amounts of acetylcarnitine and lesser quantities of propionylcarnitine were found. However, all of the tissues investigated contain several other acylcarnitines. A gas chromatographic profile of the volatile fatty acids obtained from a hydrolysate of the water-soluble acylcarnitines of beef heart is shown in Fig. 1. The prevalence of fatty acids with retention times of C4 and C5 volatile fatty acids...