Effects of toxicants on populations: A qualitative approach II. first order kinetics

System level effects exhibited by a population subjected to a chronic or an acute dose of toxicant are the emphasis of this study. A three dimensional model of a toxicant and a population, with state variables (the population biomass, the concentration of toxicant in an organism, and the concentration of toxicant in the environment) coupled by a linear dose-response function, is analyzed analytically. One of the main results presents sufficient conditions, in terms of a system level parameter, for the persistence, and for the extinction, of a population exposed to a chronic dose of toxicant. When depuration and degradation are negligible processes, the effects of toxicant accumulation associated with an acute exposure of a population are analyzed in some detail. Both persistence and extinction are shown to be viable behavior modes of a population in this biochemical setting.     

“Ischaemic” Colitis in Young Adults

Four cases of transient haemorrhagic colitis seen in young adults resemble 10 others in the literature. The 14 cases are distinguished from the classical transient ischaemic colitis syndrome by their youth, the low incidence of stricture formation, and the prevalence of right-sided lesions.     

Starch Gel Electrophoresis of Catalase and Esterase Isoenzymes from Some Rhizobium and Agrobacterium spp.

S ummary : The esterase and catalase isoenzymes of Agrobacterium tumefaciens and A. radiobacter have been compared to those of the fast growing rhizobia using starch gel electrophoresis. An overall similarity was found for the agrobacteria-rhizobia group although the majority of the agrobacteria possessed only one catalase. Those agro-bacteria with 2 catalases gave patterns similar to Rhizobium meliloti and were 3-ketolactose negative. Agrobacterium pseudotsugi isoenzymes, although fitting the general agrobacteria-rhizobia pattern, were not identical to any specific isogram. The taxonomic value of these findings is discussed.     

The structure and properties of histone F2a comprising the heterologous group F2a1 and F2a2 studied by 13C nuclear magnetic resonance

¹³C n.m.r. spectra have been obtained for aqueous solutions of histones F2a₁ and F2a₂, for the group F2a, for the appropriate amino acid mixturesand for the corresponding hydrolysates. These, when compared with computer simulated spectra give good agreement for secondary structure with that calculated from the known primary structure of the proteins. Evidence based on the spectra obtained at various salt concentrations leads to the conclusion that F2a is not a simple mixture but an interacting heterologous group of histones F2a₁ and F2a₂.     

Effects of lipids on cyclic-nucleotide phosphodiesterases

Several naturally-occurring lipids but not n-propanol, guanidine-HCl or a variety of synthetic detergents stimulate the 3′,5′-cyclic AMP-phosphodiesterase activities of a supernatant fraction of brain at 1.25 × 10^?7 M cAMP. The time courses of the reaction are linear in the presence and absence of lipid. On the other hand, lipid has different effects on various phosphodiesterase activities in fractions obtained after gel filtration of the crude extract. It stimulates the phosphodiesterase activities measured at 1.25 × 10^?7 M and 10^?4 M 3′,5′-cyclic-AMP and 1.25 × 10^?7 M 3′,5′-cyclic GMP in two of the fractions partially retained in the gel. However, lipid has little effect on the enzymatic hydrolysis of low concentrations of cAMP or cGMP and markedly inhibits the hydrolysis of high concentrations of cAMP by the fraction excluded from the gel.