L-CARNITINE MOIETY ASSAY - AN UP-TO-DATE REAPPRAISAL COVERING THE COMMONEST METHODS FOR VARIOUS APPLICATIONS

L-Carnitine and its esters are typical endogenous substances. Their ho meostatic equilibria are effectively controlled by various mechanisms which include rate-limiting enteral absorption, a multicomponent endog enous pool which is regulated according to a mammillary metabolism, an asymmetric body distribution and a saturable tubular reabsorption pro cess leading to renal thresholds. In formal pharmacokinetic and metabo lic investigations, the whole L-carnitine pool should be investigated, owing to the rapid interchange process between the various components of the pool. Free L-carnitine, as well as its acyl esters, must there fore be considered from an analytical viewpoint. L-Carnitine, acetyl-L -carnitine and total L-carnitine (the latter as an expression of the w hole pool) can easily be assayed by enzyme or radioenzyme methods. Pro pionyl-L-carnitine and other esters containing fatty acids with more t han three carbon atoms can be assayed using various HPLC approaches. T andem mass spectrometry is another excellent approach to the assay of carnitine and its short-chain, medium-chain and long-chain esters. As L-carnitine contains a chiral carbon atom, the enantioselectivity of t he assays is also considered in this review. Metabolites produced by e nteral bacteria, namely tri-, di- and mono-methylamine, gamma-butyrobe taine, along with other systemic metabolites, namely trimethylamine N- oxide and N-nitroso dimethylamine, are very important in quantitative and toxicokinetic terms and require specific assay methods. This revie w covers the commonest methods of assaying carnitine and its esters, t heir impurities and pre-systemic and systemic metabolites and gives an alytical details and information on their applications in pharmaceutic s, biochemistry, pharmacokinetics and toxicokinetics. (C) 1997 Elsevie r Science B.V.