Carnitine is widely distributed in the tissues of animals, plants and microorganisms. However, whilst there is an abundance of literature on the role of carnitine in animal metabolism, its role in plant metabolism has been less well studied. This chapter reviews work carried out on carnitine in plants and discusses its possible roles in plant metabolism, in particular the shuttling of activated acyl groups between membranebound organelles.

General background

Carnitine (3-hydroxy-4-N-trimethyl ammonium butyrate) is a highly polar compound that is widely distributed in nature. It was discovered over 85 years ago independently by two groups, Kutscher (1905) and Gulewitsch & Krimberg (1905). The latter group found the empirical formula to be C7H15NO3. Crawford & Kenyon (1927) identified its structure as being (CH3)3N+CH2CHOHCH2COO-. Carnitine has two lowenergy conformers; an extended conformer (Fig. L4) and a folded conformer (Fig. IB). Theoretical data show that the preferred conformation for carnitine is the extended form (Murray et al., 1980).

The carbon atom at the 3 position is an asymmetric carbon atom; thus, carnitine exists as D- and L-optical isomers, the L-form being the naturally occurring, biologically active isomer.

After being considered for many years as merely a constituent of vertebrate muscle, a role for carnitine was established when Fraenkel & Blewett (1947) demonstrated that it is an essential growth factor for larvae of the mealworm, Tenebrio molitor. Parallel investigations by Bremer's group (Bremer, 1983) and Fritz's group (Fritz, 1963) established a role for carnitine in the P-oxidation of long-chain fatty acids. In mammals carnitine is not metabolised, except by bacteria in the gut (Bieber, 1988).