Succession processes in a food web of a two autotroph--one herbivore system.

This paper deals with the succession process of a food web model consisting of one herbivore, two autotrophs and available nutrient in the environment in a closed nutrient flux. The model provides a way of describing successional changes in the form of species replacement with increasing nutrient levels. It is shown that distinct threshold (with upper and lower) values of nutrient are required for progression of succession process.

Microbial degradation of the plant sulpholipid.

Five bacterial strains capable of using sulphoquinovose (6-deoxy-6-sulpho-D-glucopyranose) as a sole source of carbon have been isolated and partially characterized.

Re-evaluation of plant sulpholipid labelling from UDP-[14C]glucose in pea chloroplasts.

The sulphoquinovosyldiacylglycerol (sulpholipid) zone obtained by TLC of pea chloroplast lipids yields, on hydrolysis, not only sulphoquinovose but also galactose and glucose. Following incorporation from UDP-[(14)C]glucose, the percentages of the total radioactivity in these three sugars were typically 1, 5 and 85%, respectively. The occurrence of the glucolipid causes difficulties in elucidating further the biosynthetic pathway for sulphoquinovose.

On local(ly) ESS of a pair of prey-predator system with predatory switching.

This paper concentrates on the study of ecological stability for guaranteeing evolutionary stable strategies (ESS) in a two pre-predator system taking into consideration of handling time, with predatory switching. Here predators are polyphagous in nature. The conditions for ESS of the model system are obtained at the equivalence point. We also derive the invasion conditions of a mutant predator.

Effect of time lag on non-living resource in a simple food chain.

This paper deals with a system of one non-living resource detritus, detritivores and their predators where time delay has been considered in the detritus formation. Local stability criteria are derived in the absence of delays. Conditions are derived under which there can be no change of stability whatever the value of time lag and finally we obtain criteria under which the system admits a finite number of stability switchings as the time lag increases.

A new pathway for the synthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol.

A new pathway is proposed for the biosynthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol. The pathway begins at UDP-glucose and involves the formation therefrom of UDP-4-ketoglucose-5-ene to which is subsequently added sulphite (or its metabolic equivalent). Evidence consistent with this pathway, rather than with the previously proposed 'sulphoglycolytic' route, was obtained from experiments with pea chloroplast preparations. The evidence included the failure of potential inhibitors of the sulphoglycolytic pathway to alter the rate of synthesis of sulpholipid and the stimulation of the incorporation of 35SO4(2-) into the latter by UTP. Radioactivity was effectively incorporated into sulpholipid from UDP-[14C]glucose and this radiolabelling was stimulated by the addition of methyl alpha-glucose-enide or of an enzyme system known to be forming (although not accumulating) UDP-4-ketoglucose-5-ene. This new pathway is also consistent with other data in the literature.

Persistence of a generalized gause-type two prey-two predator pair linked by competition.

We consider a generalized Gause-type model of a two predator-two prey system in which the predators are prey-specific and the prey are in competition with each other. Uniform persistence, weak persistence, and the existence of different types of heteroclinic cycles with multiple boundary equilibria are discussed.

Multiple cyclic sets connecting saddle points and limit cycles of a four-species system and its uniform persistence.

Considering a model of a generalized Gause-type four-species system of two predator-prey pairs linked by competition, the authors derive sufficient conditions for the existence of three different invariant cyclic sets connecting saddle points and limit cycle(s) on the boundary of its state space. Also discussed is the uniform persistence of the model in the presence of such cycle sets.

On the reactions of the carbapenem, meropenem, in the soluble fraction of rat kidney.

1. When meropenem (I) is incubated with the soluble fraction of rat kidney or its ultrafiltrate the carbapenem disappears at a rate of 50 nmol/min per g of kidney at pH 8.4 and 37 degrees C. 2. In both cases the kinetics are inconsistent with the reaction being enzymic and the product has been shown to be a thiol. 3. The same reaction occurs in similar preparations from the kidney of several other mammalian species. 4. The reaction of meropenem with thiols has been investigated: only those having a vicinal amino group react rapidly. 5. The product of the reaction with cysteine has been characterized as the amide(III) of the latter with meropenoic acid (II), the carboxylic acid formed by the hydrolysis of the beta-lactam ring of meropenem. 6. It is postulated that the destruction of meropenem in the soluble fraction of rat kidney occurs by the non-enzymic reaction of the carbapenem with endogenous cysteine or related thiols. 7. There is no evidence that this reaction can occur in vivo but it should be considered in the future design of carbapenems.

A co-evolutionary model of mutualism from a commensal association on Lotka-Volterra dynamics.

This paper considers a Lotka-Volterra type of model of competition between a commensal pair of species and a mutualistic pair, presumed to have descended from a single ancestral pair of species. The species are behaviourally isolated but compete for resources. We have studied in detail the effects of the benefits of mutualism over commensalism. Specifically, it is shown that co-evolution of the mutalistic pair from the commensal association is not possible if (i) some of the interacting parameters coincide for the two pairs, or (ii) mutualists suffer more from competition, or (iii) mutualists produce a smaller competitive effect. If the costs of mutualism are density-independent, co-evolution of the mutualism is possible provided (i) the specific growth rate(s) of one or both of the mutualistic species is positive at the steady state of the commensal pair and (ii) the specific growth rate(s) of one or both of the commensal species is positive at the steady state of the mutualistic pair. Lastly, whenever the mutualists co-exist with the commensalists and the feasible interior equilibrium of the model is unstable, we have shown that the co-evolutionary model of mutualism collapses to the stable two-species mutualistic relationship, causing the ultimate extinction of both the species of the original commensal association.

Stability and oscillations of a negative feedback delay model for the control of testosterone secretion.

A modified time-lag model for the control of the secretion of the hormone testosterone is proposed. Conditions for switching of the steady state from stable to unstable are derived. Questions concerning small amplitude oscillations of solutions are addressed analytically and numerically.

Persistence of a four species food chain with full omnivory.

We consider here a food chain composed of four species with full omnivory with Lotka-Volterra dynamics. Conditions for uniform persistence and weak persistence of the food chain are derived. It is also shown that the system exhibits a heteroclinic cycle.

The effect of diffusion on two predators exploiting a resource.

This paper deals with the stabilizing effect of diffusion on a system of a living resource supporting two noncompeting predators. It is shown that while the three species mathematical model without diffusion is unstable, the model with diffusion is asymptotically stable provided the diffusion coefficient of the resource exceeds a certain threshold value.

Global stability of prey-predator systems with predatory switching.

This paper deals with a complex prey-predator system, consisting of two prey species and two types of predator (dominant and mutant of the same species) with predatory switching. We derived conditions for existing polymorphism with respect to a switching property and found a condition for feasible equilibrium to be globally asymptotically stable. We have shown parameter regions for a stable and an unstable feasible equilibrium.

Aluminofluorides and beryllofluorides as inhibitors of sulphatases. Analogues of hydrogen sulphate?

The inhibition of fluoride of sulphatase A from ox liver and of the sulphatases of Helix pomatia and Aspergillus oryzae is decreased by EDTA and increased by Al3+ or Be2+, implicating aluminofluorides and beryllofluorides in the reaction. The inhibition, which is reversible, takes several minutes to develop fully and, at least for the sulphatase of H. pomatia, is of a non-linear mixed competitive-non-competitive type. It is suggested that the aluminofluorides and beryllofluorides are acting as analogues of HSO4-. If so, then this behaviour must be considered, as well as their role as analogues of phosphate, in interpreting the effects of these compounds in intact cells.

Sulphation of the flavonoids quercetin and catechin by rat liver.

1. The isolated perfused rat liver forms three sulphated metabolites from each of the flavonoids, quercetin and catechin: these are secreted into the bile and the perfusate. 2. Quercetin gives two double conjugates, containing sulphate and glucuronic acid, and one sulphate: catechin gives one such double conjugate and two sulphates. 3. This sulphation is not inhibited by 60 microM 2,6-dichloro-4-nitrophenol which almost completely inhibits the sulphation of harmol in this perfused liver system. 4. The sulphation of harmol by the perfused liver is not inhibited by the flavonoids. 5. Unfractionated sulphotransferases from rat liver catalyse sulphate conjugation of quercetin and catechin in vitro by a reaction inhibited by pentachlorophenol or dichloronitrophenol: the flavonoids inhibit the sulphation of 4-nitrophenol by this system. 6. The results with the two systems are discussed and shown to be compatible.

The uptake of oestrone from the lumen of the isolated perfused rat gut.

1. In the isolated perfused rat gut oestrone was not taken up from the perfusate but was rapidly taken up from the gut lumen when administered in dimethyl sulphoxide or in 10% acetone in water. 2. Uptake from corn oil was much slower and the rate was concentration-dependent. 3. When given in dimethyl sulphoxide or in aqueous acetone, the residual oestrone in the gut lumen was largely unchanged, with only about 10% being present as the glucuronide. Of the oestrone in the gut perfusate, some 40% was glucuronidated. 4. No evidence was obtained for any reduction or hydroxylation, or for any sulphation, during uptake of oestrone by the gut. 5. No secretion of oestrone or its metabolites into lymph could be detected.

Uniform persistence and global stability of two prey-predator pairs linked by competition.

General models of two predator-prey systems are considered in which the prey are linked through competition and the predators are not directly linked. The persistence criteria based upon a technique developed by Gard is obtained. In addition, a condition for the global asymptotic stability of the interior equilibrium is discussed.

The inhibition by chlorate of the sulphation of polyethyleneglycol in the isolated perfused guinea pig liver.

1. The sulphation of polyethyleneglycol 200 by the isolated perfused guinea pig liver is inhibited to about 60% by 10 mM ClO3- in the plasma of the perfusate when the concentration of SO4(2-) therein is 1.18 mM. 2. The inhibition is almost complete when the concentration of SO4(2-) is about 0.1 mM, a level which can be achieved by using a modified Ringer-bicarbonate solution, devoid of sulphate, to prepare the perfusate. 3. Chlorate, presumably through its action on ATP-sulphurylase, may therefore be a useful inhibitor of sulphation in the isolated perfused liver when the activity of the sulphurylase is rate-limiting. 4. The rate of bile production in the presence of chlorate is no different from that in its absence showing that, in the time scale of the perfusion, chlorate is not a general liver poison. 5. When the synthesis of PAPS is not rate-limiting, as in the sulphation of oestrone metabolites by rat liver, chlorate has no effect on the rate of sulphation.