Abstract
Considerable amounts of GTP, GMP, adenosine, guanine, and adenine accumulated in Neurospora crassa when a culture grown on low phosphate (0.01%) medium was transferred to a high phosphate (1%) medium. The levels of alkaline phosphatase, nucleotidase, and nucleosidase decreased by 2.4, 5.4, and 3 folds, respectively, in cultures grown on high phosphate medium. Substrate kinetics of these enzymes revealed that: (1) alkaline phosphatase isolated from the organism grown on low phosphate medium demonstrates nonlinear reciprocal plots with two distinct apparent Km values for Ξ²βglycerophosphate compared to one apparent Km value (associated with 32% decrease in the apparent V~max~ value) obtained with that grown on high phosphate medium; (2) nucleotidases and nucleosidases isolated from organisms cultivated on low phosphate and high phosphate media showed the same apparent Km values, 0.25 mM for nucleotidase and 0.909 mM for nucleosidase. There was, however, >3 times decrease in the catalytic activity of the latter enzymes isolated from organisms grown on high phosphate medium as compared to those grown on low phosphate. Inclusion of inorganic phosphate in standard assay mixtures of the three enzymes resulted in a considerable inhibition in the catalytic activities of all of them. High levels of phosphate in the medium caused marked repression of three out of six of alkaline phosphatase, two out of three of nucleotidase, and one out of two of nucleosidase isozymic forms detected in the low phosphate grown culture.