Enzymes of glycolysis, pentose phosphate pathway, gluconeogenesis, tricarboxylate acid cycle, glyoxylate by-pass and fatty-acid biosynthesis were assayed in extracts from Candida 107 grown continuously on glucose under carbon limitation, nitrogen limitation and on n-alkanes. The yeast was therefore either in a lipogenic or lipolytic state. Phosphofructokinase was absent under all conditions whereas enzymes of gluconeogenesis, including fructose I ,6-bisphosphatase and the pentose phosphate cycle, were all present. Glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were specific for NADP+ and were inhibited in a non-competitive manner by NADPH and NADH. Phosphoenolpyruvate, citrate, ATP and acetyl CoA had no inhibitory effects. Thus glucose metabolism appears to be by the pentose phosphate pathway which will rapidly produce NADPH. This can readily be consumed during fatty-acid biosynthesis and, as there appears to be no inhibition of the flow of carbon from glucose to acetyl CoA, fatty-acid synthesis can continue for as long as there is a supply of glucose. These results help to explain the probable causes of fat build-up to high concentrations (about 40 7; of the cell dry weight) in this and other organisms. In alkane-grown cells, lipogenesis is repressed and carbon is able to flow from the alkanes via acetyl CoA, oxaloacetate and pyruvate into pentoses and hexoses in a unidirectional manner, because of the strong repression of pyruvate kinase and the increased activities of phosphoenolpyruvate kinase and fructose I ,6-bisphosphatase under these conditions. Although there was little change in the total activity of the TCA cycle enzymes under the various growth conditions, isocitrate lyase was induced under lipolytic conditions.

SummaryOxidation rates of several alkane substrates by C. lzpolytzca ATCC 8661 grown on n-dodecane were determined using a Warburg Respirometer. Substrates were emulsified using Span 20, Span 80, Tween 20, Tween 80, and Triton X-100 surfactants and the effects of these surfactants on oxidation and growth were determined. The oxidation rates of a number of intermediates, including lauric acid and lauryl alcohol, were also assessed. Responses of dodecane-grown C . lipolytica to select substrates were compared to the corresponding behavior with glucose-grown yeast and with baker's yeast. The role of surfactants in hydrocarbon fermentations is discussed in the light of the present and previously published data.

397Phosphoketolase(s), catalysing the cleavage of xylulose 5-phosphate and fructose 6-phosphate, were found in Rhodotorula graminis, R. glutinis, Candida 107 and C. tropicalis. Activity towards the latter substrate only was found in C. humicold. Neither activity was found in Sdccharomyces carlsbergensis. The enzyme(s) were isolated and purified eightfold from R. graminis. Inorganic phosphate, thiamin pyrophosphate and Mg2+ were required for activity. The enzyme had a K , of 1-25 mM for xylulose 5-phosphate and was markedly sensitive to NADH, NADPH, ATP and acetyl-CoA and less sensitive to phosphoenolpyruvate, citrate and dodecanoyl-CoA. The activity of phosphoketolase was halved in the presence of an active transketolase which competes with it for the same substrate. The contribution of phosphoketolase to glucose catabolism may be only slight.

Candida lipolytica (strain ATCC 8661) was grown on a simple defined medium with n-dodecane as sole carbon source under batch and continuous fermentation conditions. The composition of cellular material recovered from the fermentations, the oxygen demand of the cells, and the effect of operating conditions on cell growth were evaluated experimentally. These basic data are presented and discussed.

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