Studies on marine occurring yeasts: Relations to inorganic nitrogen compounds, especially hydroxylamine

Changes in free amino acid content and activities of amination and transamination enzymes in yeasts grown on different inorganic nitrogen sources, including hydroxylamine

This study concerns inter- and intraspecific differences between yeasts at assimilation of different nitrogen sources. Alterations in the content of free amino acids in cells and media as well as in the related enzyme activities during growth were studied. The hydroxylamine (HA)-tolerant Endomycopsis lipolytica was examined and compared with the nitrate-reducing Cryptococcus albidus, and Saccharomyces cerevisiae, requiring fully reduced nitrogen for growth. Special attention was paid to alanine, aspartic acid, and glutamic acid, the amino acids closely related to the Krebs cycle keto acids. The amino acids were analyzed as their n-propyl N-acetyl esters by gas-liquid chromatography (GLC). The composition of the amino acid pool was similar for the three yeasts. Glutamic acid was predominant; in early log-phase cells of E. lipolytica contents of 200-234 micromol . g(-1) dry weight were found. A positive correlation between the specific growth rate and the size of the amino acid pool was observed. The assimilation of ammonia was mediated by glutamate dehydrogenase (GDH). The NADP-GDH was the dominating enzyme in all three yeasts showing the highest specific activity in Cr. albidus grown on nitrate (6980 nmol . (min(-1)).(mg protein(-1)). Glutamine synthetase (GS) displayed a high specific activity in S. cerevisiae, which also had a high amount of glutamine. The assimilation of HA did not differ greatly from the assimilation of ammonium in E. lipolytica. The existing differences could rather be explained as provoked by the concentration of available nitrogen.

Cellular content of the Krebs cycle keto acids in yeasts grown on different nitrogen sources, including hydroxylamine

The cellular pool of Krebs cycle keto acids was followed as a function of growth in three yeasts. The keto acids were analyzed as silylated methoximes by quantitative gas chromatography with capillary glass columns. The 2-oxoglutaric acid content was strikingly high in the hydroxylamine (HA)-tolerant, HA-utilizing Endomycopsis lipolytica when compared to that in the nitrate-utilizing yeast Cryptococcus albidus and Saccharomyces cerevisiae, requiring fully reduced nitrogen for growth. The content of E. lipolytica increased throughout the log phase to maxima of about 200-250 microgram per g dry weight in HA and ammonia media. These amounts are 20-25 times greater than those attained in the two other yeasts. The cellular content of pyruvic acid was at a maximum early in the log phase, amounting to 50-70 microgram per g dry weight for all yeasts. The oxalacetic acid content never exceeded 9 microgram per g dry weight in any of the yeasts. Oximeformation, for which keto acid production is a prerequisite, is discussed as part of the HA-tolerance.