Effect of ethanol on the specific transport system for L-lysine inSaccharomyces cerevisiae

Preincubation of resting cells of Saccharomyces cerevisiae double mutant can1 gap1 (with a single transport system for L-lysine) with metabolic substrates stimulated subsequent uptake of lysine. While in the wild type the stimulation is connected primarily with carrier protein synthesis (delayed, cycloheximide-inhibitable effect) in the mutant an immediate tapping of an energy source (antimycin-inhibited) is practically solely involved.     

Photosynthetic electron transport interaction of xanthorrhizol isolated from Iostephane heterophylla and its derivatives

A bioactivity-guided chemical study of Iostephane heterophylla (Asteraceae) led to the isolation of xanthorrhizol (1) as the compound that causes inhibition of ATP synthesis, H⁺-uptake and electron flow from water to methylviologen (basal, phosphorylating and uncoupled) in freshly lysed spinach chloroplasts, thus acting as an inhibitor of the Hill reaction. Acetyl (2), dihydro (3) and acetyl-dihydro (4) derivatives were synthesized. It was found that 4 was less active than 1 and 2 in ATP synthesis, whereas 3 was the most potent inhibitor of the Hill reaction and was also an inhibitor of H⁺-ATPase. Studies of the photosynthetic partial redox reactions from PQ to MV indicated that 1 partially inhibited the PQ pool, but that 3 did not. However, both inhibited the uncoupled electron transport in PSII from water to DCBQ. Uncoupled electron flow from water to silicomolybdate was completely inhibited by 3 and partially by 1. The reaction from DPC to DCPIP was inhibited by both 1 and 3. These results indicate that the inhibition site is located within PSII for 1 and 3 as was corroborated by fluorescence decay data.     

Characterization of the Actions of Toxins II-9.2.2 and II-10 from the Venom of the Scorpion Centruroides noxius on Transmitter Release from Mouse Brain Synaptosomes

Toxic peptides II-9.2.2 and II-10, purified from Centruroides noxius venom, bear highly homologous N-terminal amino acid sequences, and both toxins are lethal to mice. However, only toxin II-10 is active on the voltage-clamped squid axon, selectively decreasing the voltage-dependent Na+ current. Here, we have tested toxins II-9 and II-10 on synaptosomes from mouse brain: both toxins increased the release of gamma-[3H]aminobutyric acid ([3H]GABA). Their effect was completely blocked by tetrodotoxin or by the absence of external Na+. Also, both toxins increased Na+ permeability in isolated nerve terminals. Besides the observation that toxin II-9 is active on synaptosomes, the effect of toxin II-10 in this preparation is opposite to that observed in the squid axon. Thus, our results reflect functional differences between the populations of Na+ channels in mouse brain synaptosomes and in the squid axon. The release of GABA evoked by these toxins from synaptosomes required external Ca2+ and was blocked by Ca2+ channel blockers (verapamil and Co2+). This latter observation is in sharp contrast to the releasing action of veratrine, which evoked release even in the absence of external Ca2+. Furthermore, the action of both C. noxius toxins was potentiated by veratrine, a result suggesting they have different mechanisms of action. Among drugs that release neurotransmitters by increasing Na+ permeability, it is noteworthy that scorpion toxins are the only ones yet reported to have a strict requirement for external Ca2+.     

Changes in histones H2A and H3 variant composition in differentiating and mature rat brain cortical neurons

Rat brain cortical neurons originate from germinal cells during a period of 6 days immediately before birth. Upon leaving the proliferative layer neurons become irreversibly quiescent. We have previously reported the presence of core histone nonallelic variants in terminally differentiated rat brain cortical neurons. Although the functional significance of core histone variants is unknown, several lines of evidence suggest that the processes of variant replacement could be involved in the structural and functional differentiation of chromatin. Here we describe the changes in core histone composition that occur during postnatal development. The changes in chromatin composition are already apparent at birth, suggesting that the change in synthesis patterns is related to the arrest of cell proliferation and neuron commitment. During postnatal development H2A.2, H2A.x, and H3.3 accumulate, whereas H2A.1, H3.1, and H3.2 decrease. H2A.z is the only variant that remains constant. The time courses of replacement and the observed variant proportions when the variant composition approaches the equilibrium suggest that all H2A variants are synthesized either in germinal cells or in neurons, whereas H3.1 and H3.2 seem to be synthesized only in germinal cells. The extent of the replacement of H3.1 and H3.2 by H3.3 shows that the exchange process affects most of the chromatin. The half-life times of H2A.1 and H3.2 were calculated from their respective exponential decays. Values of 65 days or less and 142 days were found for H2A.1 and H3.2, respectively. The preferential replacement of H2A.1 over H3.2 reinforces the view that the histone core does not degrade as a single unit.     

Changes in H1 complement in differentiating rat-brain cortical neurons

Neuronal nuclei have a low H1 content. A stoichiometry of 0.47 molecule/nucleosome, on average, is calculated for rat brain cortical neurons by comparing its H1 content with that of liver nuclei. The H1 fraction of rat cerebral cortex neurons has been resolved into five subtypes, H1a--e, that have the same mobility as the unphosphorylated H1 forms of other rat tissues. The subtypes H1a--d decay exponentially during postnatal development and are substituted to different extents by H1e. The higher replacement rate is shown by H1a with an apparent half-lifetime of about 5 days. The corresponding values for H1b, H1c and H1d are 11, 21 and 15 days. Several conclusions can be drawn from the observation of postnatal changes in H1 subtype proportions. The low H1 content of neuronal nuclei does not imply the presence of notable peculiarities in subtype composition or in subtype substitution pattern. There is turnover of H1 in differentiating neurons once cell proliferation and DNA replication have ceased. The relative rates of synthesis and/or degradation of the subtypes differ in germinal cells and in neurons. Comparison with previous results on H1 degrees accumulation also shows that in cortical neurons the regulation of the subtypes H1a--e differs from that of H1 degrees.     

Differences in the half-lives of some mitochondrial rat liver enzymes may derive partially from hepatocyte heterogeneity

The different turnover rates of rat liver mitochondrial enzymes make autophagy unlikely to be the main mechanism for degradation of mitochondria. Although alternatives have been presented, hepatocyte heterogeneity has not been considered. Lighter hepatocytes isolated in a discontinuous Percoll gradient contain more glutamate dehydrogenase (GDH) (half-life 1 day) and a more active autophagic system than heavier hepatocytes. The latter contain more carbamoyl phosphate synthase (CPS) and ornithine carbamoyl transferase (OTC) (half-lives 8 days) but less lysosomal activity. As expected, isolated autophagic vacuoles contain, relative to the mitochondrial content, 3-times less OTC and CPS than GDH, probably reflecting a faster lysosomal engulfment of mitochondria in the light hepatocytes (which contain more GDH). These data may explain some of the half-life differences of the enzymes studied.