Kinetic properties of rat liver pyruvate kinase type I at pH7.5 and 6.5 were studied with physiological ranges of substrates, modifiers and Mg(2+) concentrations at increasing enzyme concentrations, including the estimated cellular concentrations (approx. 0.1mg/ml). Enzyme properties appear unaffected by increased enzyme concentration if phosphoenolpyruvate, fructose 1,6-diphosphate and inhibitors are incubated with enzyme before starting the reaction with ADP. Our data suggest that minimum cellular concentrations of MgATP and l-alanine provide virtually complete inhibition of pyruvate kinase I at pH7.5. The most likely cellular control of existing pyruvate kinase I results from the strong restoration of enzyme activity by the small physiological amounts of fructose 1,6-diphosphate. Decreasing the pH to 6.5 also restores pyruvate kinase activity, but to only about one-third of its activity in the presence of fructose 1,6-diphosphate. Neither pyruvate nor 2-phosphoglycerate at cellular concentrations inhibit the enzyme significantly.

No abstract

No abstract

Intestinal basement membrane from the helminth, Ascaris suum, was isolated free of cellular material by ultrasonic treatment and exhaustive washing. The basement membrane is 3.0 to 4.0 jim thick. Sections of the sonicated basement membrane examined by light and electron microscopy show a thick and thin sublamina separated by a line of denser material. The bulk of the material in the two sublamina is a fine feltwork with no identifiable strands of collagen fibrils. It is composed of 4.9% carbohydrate and 91% amino acids by weight. The monosaccharides were identified as galactose, glucose, mannose, fucose, glucosamine, and galactosamine. In contrast to basement membranes of vertebrate sources, there was no detectable sialic acid. The amino acid composition is dominated by glycine, which -Basement membranes are extracellular membranes which are widely distributed in animal tissue and function as tissue support and in certain locations serve as a selective ultrafilter . These membranes are considered to be a specialized form of collagen since they exhibit wideangle X-ray diffraction patterns and possess several chemical properties which are characteristic of collagen (Kefalides and Winzler, 1966;Kefalides and Denduchis, 1969; Denduchis and Kefalides, 1970;.The amino acid and carbohydrate composition has been determined for several vertebrate basement membranes including the glomerular (