Dynamics of some special populations with NRR = 1

We discuss two special cases of population dynamics with changing vital rates subject to the constraint NRR = 1 and consider the specifics of ergodicity and stationarity. By restricting the number of age groups to two, we can make explicit the way weak ergodicity works. When mortality is constant and the period NRR = 1, a life-table stationary population eventually results; everything of demographic interest (except age-specific fertility and the cohort NRR) is asymptotically constant. With changing mortality, but both period and cohort NRR = 1, only weak ergodicity holds, and everything changes over time.

Genetic variation in irradiated and control populations of Cnemidophorus tigris (sauria, teiidae) from Mercury, Nevada with a discussion of genetic variability in lizards

Whiptail lizards (Cnemidophoms tigris) were collected from fenced irradiated, fenced control, and unfenced areas near Mercury Nevada. No changes in allele frequencies at 26 allozyme loci could be ascribed to irradiation or fencing. This species is the most polymorphic and heterozygous lizard so far examined. - Heterozygosity estimates derived from electrophoretic studies on 20 additional species of lizards are compared with Cnemidophorus. A general trend seems to emerge. Fossorial lizards have uniformly low levels of heterozygosity (ca. 1 %). Territorial "sit and wait" predators are intermediate (ca. 5%). Highly vagile apparently nonterritorial lizards are the most heterozygous (ca. 10%). Assuming that this trend does not reflect some of sampling error, two current, non-mutually exclusive hypotheses explain the observed situation: (1) the niche width variation hypothesis predicts, higher variability in populations where individuals are exposed to largescale environmental heterogeneity; and (2) the population size hypothesis predicts that, all other things being equal, vagility would tend to increase the effective population size by reducing inbreeding, which would promote higher levels of genetic variation.

On the mechanism of pyridoxine responsive homocystinuria. II. Properties of normal and mutant cystathionine beta-synthase from cultured fibroblasts

Cystathionine beta-synthase [L-serine hydrolyase (adding homocysteine), EC 4.2.1.22] was studied in cultured skin fibroblasts from two control subjects and three patients with pyridoxine-responsive homocystinuria. In crude cell sonicates, cystathionine synthase activity detected in each mutant line was less than 5% of control values. After differential centrifugation, ammonium sulfate fractionation, and calcium phosphate gel treatment, the specific activity of synthase from control lines increased 5- to 7-fold with 70-79% yield. These same steps led to only 2- to 3-fold purification of mutant synthase and a reduced yield (26-44%). Michaelis-Menten analyses with the partially purified enzyme revealed that each mutant synthase had a marked reduction in affinity for its coenzyme, pyridoxal 5'-phosphate, as well as reduced affinity and maximum velocity for both co-substrates, L-homocysteine and L-serine. Even at saturating concentrations of coenzyme, mutant synthase activity was less than 3% of control. Mutant synthase was also far more thermolabile than control enzyme. In the absence of added coenzyme, heating for 10 min at 55 degrees led to complete loss of mutant activity whereas control activity was reduced by 60%. Significantly, addition of saturating concentration of coenzyme prior to heating increased thermostability of both control and mutant synthase, the fractional increase being considerably greater in the mutants. We conclude that these patients suffer from a mutation of the synthase apoenzyme which impairs coenzyme binding, and that this primary abnormality results in reduced total enzyme activity in two ways: by reducing holoenzyme formation; and by accelerating apoenzyme degradation. We propose that pharmacologic amounts of pyridoxine increase holoenzyme formation modestly, thereby enhancing catalytic activity and slowing apoenzyme turnover.

Amino acid metabolism during starvation in human pregnancy

To evaluate the factors regulating gluconeogenesis in pregnancy, plasma amino acid levels were determined during the course of an 84-90 hr fast in physically healthy women studied during wk 16-22 of gestation (before undergoing therapeutic abortion), and in nonpregnant controls. The effect of pregnancy on the glycemic response to exogenous alanine administration during starvation was also investigated. In the nonpregnant group fasting resulted in a 2- to 3-fold increase in the levels of plasma valine, leucine, isoleucine, and alpha-aminobutyrate, while the concentration of alanine and glycine fell. In the pregnant group, the levels of most amino acids were significantly reduced in the postabsorptive state. With starvation, the plasma concentration of alanine fell more rapidly in the pregnant group and was significantly below that of the nonpregnant subjects for the first 60 hr of the fast. In contrast, a significant elevation in plasma glycine, serine, and threonine was observed in the pregnant group after 84 hr of fasting, whereas similar increments were not demonstrable until after 10 days of fasting in previously studied nonpregnant obese subjects. Paralleling the changes in maternal plasma, amniotic fluid levels of valine, leucine, and isoleucine increased while that of alanine fell during the fast. Although the plasma glucose concentration was lower in the pregnant group at termination of the fast, intravenous alanine administration (0.15 g/kg), resulted in a prompt, comparable increase (20-25 mg/100 ml) in plasma glucose in both groups of subjects. It is concluded that (a) pregnancy accelerates and exaggerates the hypoalaninemic and hyperglycinemic effects of starvation; (b) lack of key endogenous substrate rather than altered intrahepatic processes may limit hepatic gluconeogenesis in pregnancy and contribute to gestational hypoglycemia; (c) maternal caloric deprivation profoundly alters the levels of amino acids in amniotic fluid.