A Mechanism of Conversion of Fat to Carbohydrate in Castor Beans

Abstract

THE mechanism of the interconversion of fat and carbohydrate, known to occur in germinating fatty seedlings, has not yet been established. It has been shown that the castor bean can utilize acetate carbons in the synthesis of sucrose, and that the labelling of the sugar after incubation of bean extracts with sodium acetate-¹⁴C is consistent with the operations of glycolysis and the tricarboxylic acid cycle¹. Yet these reactions are not by themselves sufficient to account for any net synthesis of carbohydrate from fat, but necessitate the occurrence of ancillary replacement reactions, whereby the intermediates drained from the cycle could be replaced directly by the breakdown of fatty acids. The ‘glyoxylate bypass’ of the tricarboxylic acid cycle demonstrated in acetate-grown Pseudomonas ² supplies such a replacement mechanism, which would also account for the observed labelling patterns. It was of interest, therefore, to investigate the possible occurrence of this by-pass in castor beans, particularly since malate is one of the earliest labelled products formed from labelled acetate in this tissue.