Threonine as a perferred source of 2-carbon units for lipid synthesis in Trypanosoma brucei.

Culture forms of Trypanosoma brucei S42 were maintained in defined media by serial subculturing (Cross & Manning, 1973; Klein et al., 1975). Under these conditions considerable quantities of L-threonine were metabolized and equimolar concentrations of glycine and acetate were excreted. Glycine was identified by using an amino acid analyser and acetate was characterized as the p-bromophenacyl derivative (Cross et al., 1975). The pathway by which acetate is produced from L-threonine by culture form T. brucei was shown to involve the NAD+-dependent L-threonine dehydrogenase (EC 1.1.1.103) and 'aminoacetone synthase' (McGilvray & Morris, 1969). In homogenates the enzymic sequence is interrupted and aminoacetone is produced ; this was identified as the semicarbazide (R. A. Klein & D. J. Linstead, unpublished results). The breakdown of L-threonine is shown diagrammatically in Fig. 1. Acetate produced from radioactively labelled L-threonine (1.23m~) was found to be a preferred source of lipid carbon even in the presence of both exogenous acetate (0.66m~) and a tenfold molar excess of glucose (13.2m~). The incorporation of radioactivity from labelled L-threonine into chloroform/ methanol-soluble lipid material was decreased by adding unlabelled sodium acetate (6.76m~) to the culture medium. However, the dilution effect (0.623+0.015) was very much less than expected on the basis of the relative concentrations used. This is shown in Fig. 2, where (a) represents the radioactivity incorporated from ~-[U-'~C]threonine alone, (b) from ~-[U-'~C]threonine in the presence of unlabelled sodium acetate