The kinetic model of carbohydrate metabolism has been expanded to include : (a) the accumulation of CI and /3-cellulose, insoluble cell-wall glycogen and mucopolysaccharide; (b) the role of RNA turnover as a source of carbon for end-product synthesis and as a buffer regulating the level of uridine nucleotides in this metabolic network ; and (c) the role of purine-nucleoside phosphorylase, 5'-AMP nucleotidase, nucleosidediphosphate kinase and polynucleotide phosphorylase. One of many predictions based on this model is that cells differentiating in the presence of glucose will produce sorocarps with an abnormally high trehalose to cellulose ratio. External perturbation of either the model or of developing cells by glucose increases the levels of sorocarp trehalose and glycogen, 5-fold and 6-fold respectively. Evaluation of the experimental data and the simulation analyses have allowed several predictions to be made concerning the compartmentation of metabolites and the permeability of cells to glucose during differentiation.During differentiation in many systems enzymes change in specific activity owing to a variety of mechanisms. A major problem in developmental biology is to determine which enzymes are rate-limiting and hence could participate in controlling the differentiation process in question. Such a determination can only be made by analyses permitting a simultaneous assessment of the role of other potential rate-limiting events, such as alterations in substrate and effector availability. A dynamic analysis of a metabolic network under the steady-state conditions of the living cell is possible through the use of kinetic models, which can simulate changes in enzyme activity as well as changes in metabolite flux and concentration over the course of differentiation. Such models serve to: (a) provide a framework within which to judge the relevance of data in vitro to conditions operative in the living cell; (b) organize and orient the data, hence revealing meaningful experimental approaches ;
Growth of Candida utilis and Saccharomyces cerevisiae in a medium supplemented with sulfur amino acids led to synthesis and accumulation of S-adenosylmethionine, accompanied by a reduction in the cell yield, an increased sensitivity of the cell wall to snail gut enzymes (Helix pomatia), as judged by spheroplast formation, and by a modification of the chemical composition of both the intact cells and their isolated walls. Walls of supplemented cultures of C. utilis were three times as sensitive to enzymatic digestion as walls from nonsupplemented cultures. In contrast to C. utilis, walls isolated from supplemented cultures of S. cerevisiae were digested slightly more rapidly by the purified snail extract than those from nonsupplemented cultures. Chemical modifications of the cell wall are interpreted to explain the ease with which cells from sulfur amino acid-supplemented cultures are converted to spheroplasts.Jlized by membrane filtration (type HA, 0.45 ,um pore size; Millipore Corp., Bedford, Mass.) 931 on July 10, 2020 by guest
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