Transport and metabolism of carnitine precursors in various organs of the rat.
Isolated, vascularly perfused small intestine, liver, and kidney were used to investigate their interdependence in the absorption and metabolism of carnitine precursors in the rat. During 30 min of recirculating perfusion, the small intestine absorbed trimethyllysine, hydroxytrimethyllysine, and trimethylaminobutyrate fairly well when they were administered via the lumen or the perfusate. Trimethylaminobutyrate was synthesized from either trimethyllysine or hydroxytrimethyllysine by the small intestine, but further hydroxylation of trimethylaminobutyrate to carnitine did not occur. Trimethyllysine and hydroxytrimethyllysine were not readily absorbed by the liver. In contrast, trimethylaminobutyrate and trimethylaminobutyraldehyde were rapidly absorbed from the perfusate and readily incorporated into carnitine by the liver. Trimethyllysine and hydroxytrimethyllysine were taken up slowly by the kiodney and partially converted to trimethylaminobutyrate during 3409 min of perfusion. Trimethylaminobutyrate was neither absorbed readily by the kidney nor was it hydroxylated to carnitine. These results were compared to whole animal studies performed over an equivalent time period. The data suggest that the isolted small intestine absorbs trimethyllysine well, but it probably plays a minor role in metabolizing physiological quantities of this compound in the whole animal where other organs are competing for the same substrate. In both the isolated organ and in the whole animal, the kidney absorbs and metabolizes trimethyllysine more readily than the liver; whereas the liver absorbs trimethylaminobutyrate more rapidly than either the kidney or the small intestine and, unlike these organs, converts it to carnitine.