No abstract

Adipose tissue plays important roles in regulating carbohydrate and lipid homeostasis, though less is known about the regulation of amino acid metabolism in adipocytes. Here we applied isotope tracing to pre–adipocytes and differentiated adipocytes to quantify the contributions of different substrates to tricarboxylic acid metabolism and lipogenesis. In contrast to proliferating cells that use glucose and glutamine for acetyl–coenzyme A (AcCoA) generation, differentiated adipocytes increased branched chain amino acid (BCAA) catabolic flux such that leucine and isoleucine from media and/or protein catabolism accounted for as much as 30% of lipogenic AcCoA pools. Medium cobalamin deficiency caused methylmalonic acid accumulation and odd–chain fatty acid synthesis. B12 supplementation reduced these metabolites and altered the balance of substrates entering mitochondria. Finally, inhibition of BCAA catabolism compromised adipogenesis. These results quantitatively highlight the contribution of BCAAs to adipocyte metabolism and suggest that BCAA catabolism plays a functional role in adipocyte differentiation.

In Krebs-Ringer phosphate buffer, the rate of formation of methylglyoxal from glycerone phosphate and glyceraldehyde 3-phosphate was first order with respect to the triose phosphate with rates constant values of 1.94 2 0.02X10-5 s-' (n = 18) and 1.54 2 0.02X10-4 s-' (n = 18) at 37"C, respectively. The rate of formation of methylglyoxal from glycerone phosphate and glyceraldehyde 3-phosphate in the presence of red blood cell lysate was not significantly different from the nonenzymatic value ( P > 0.05). Methylglyoxal formation from glycerone phosphate was increased in the presence of triose phosphate isomerase but this may be due to the faster non-enzymatic formation from the glyceraldehyde 3-phosphate isomerisation product. For red blood cells in vitro, the predicted non-enzymatic rate of formation of methylglyoxal from glycerone phosphate and glyceraldehyde 3-phosphate may account for the metabolic flux through the glyoxalase system. The reactivity of glycerone phosphate and glyceraldehyde 3-phosphate towards the non-enzymatic formation of methylglyoxal under physiological conditions suggests that methylglyoxal formation is unavoidable from the Embden-Meyerhof pathway.Methylglyoxal (2-oxopropanal) is a metabolite found widespread throughout biological life (Thornalley, 1990 ;Murata et al., 1989;Ohmori et al., 1989;Steinberg and Kaplan, 1984). It was initially thought to be involved in mainstream glycolysis but the discovery of phosphorylated glycolytic intermediates led to its demise as a metabolite of major importance. It has been most widely investigated with respect to the glyoxalase system which catalyses the conversion of methylglyoxal to D-lactate via the intermediate S-(D1actoyl)glutathione (Thornalley, 1990) but it is now receiving renewed interest as a substrate for aldose reductase (Vander Jagt et al., 1992). The toxicity of high doses of methylglyoxal to biological tissue prompted Albert Szent-Gyorgyi to suggest that methylglyoxal was a 'growth-inhibiting factor' or 'retine', and the glyoxalase system a 'growth-promoting factor' or 'promine', and the conflict between these two factors was involved in control of cell growth (Szent-Gyorgyi, 1977). This hypothesis has been superseded by the discovery of oncogenes and peptide factors (Bradshaw and Prentice, 1987), although recent evidence suggests that methylglyoxal, by conversion to S-(D-lactoyl)glutathione, may influence cell Correspondence to P. J. Thornalley,