Ketose induced respiratory inhibition in isolated hepatocytes

The addition of 10 mM fructose or 10 mM tagatose to a suspension of hepatocytes caused respiratory inhibition, whereas no change in oxygen uptake was observed following the addition of glucose. However, incubations in the presence of fructose showed a high, aerobic glycolytic activity. Tagatose is phosphorylated to tagatose 1-phosphate but is not further metabolized by cell free liver extract. Moreover, the addition of fructose to glucagon treated cells also caused the Crabtree-like effect. The concentration of adenine nucleotides and inorganic phosphate (Pi) in the mitochondrial and cytosolic compartments during incubation (time 30 min) was determined by the digitonin fractionation procedure. In the presence of 10 mM fructose or tagatose, the total adenine nucleotide pools decreased by 40%; however, glucose produced no change. The addition of ketoses diminished the asymmetric distribution of extramitochondrial (ATP/ADP)e ratio and intramitochondrial (ATP/ADP)i ratio. At the same time the total mitochondrial Pi fell from 17 mM to 6-7 mM. The mitochondrial membrane potential (-161 mV) in the presence of fructose showed no changes during the 30 min experimental period. An increase in the NADH/NAD+ ratio was observed. These results suggest that in hepatocytes the inhibition of respiration is not necessarily linked with the enhanced aerobic glycolysis, by competition for common substrates.

An Mr 180,000 protein is an endogenous substrate for the insulin-receptor-associated tyrosine kinase in human placenta

The beta-subunit of the insulin receptor contains a tyrosine-specific protein kinase. Insulin binding activates this kinase and causes phosphorylation of the beta-subunit of the insulin receptor. It is believed that phosphorylation of other proteins might transmit the insulin signal from the receptor to the cell. In the present study we used a polyclonal anti-phosphotyrosine antibody to detect other proteins that become tyrosine phosphorylated upon insulin stimulation. Glycoproteins from human placenta membranes were enriched by wheat germ agglutinin chromatography and phosphorylation was studied with [gamma-32P]ATP and insulin in vitro. Phosphorylated proteins were immunoprecipitated by antibodies against the insulin receptor and by serum containing the anti-phosphotyrosine antibody. Beside the insulin-stimulated phosphorylation of the 95 kDa beta-subunit of the insulin receptor, an insulin-stimulated phosphorylation of a 180 kDa protein was found. The phosphorylation of both proteins occurred only on tyrosine residues. Insulin increased 32P incorporation into the 180 kDa band 2.7-fold (S.E.M. +/- 0.3, n = 5). The 180 kDa protein was not precipitated by antibodies against the insulin receptor. H.p.l.c. chromatograms of tryptic fragments of the phosphorylated 180 kDa protein and of the beta-subunit of the insulin receptor revealed different patterns for both proteins. Insulin-stimulated phosphorylation of the 180 kDa protein was also detectable in unfractionated detergent-solubilized membranes. The phosphorylation of the 180 kDa protein was stimulated by insulin with the same dose-response curve as the phosphorylation of the beta-subunit, suggesting that this protein might be another endogenous substrate of the insulin receptor kinase.

Evidence that (a) serine specific protein kinase(s) different from protein kinase C is responsible for the insulin-stimulated actin phosphorylation by placental membrane

Partially purified phospholipid- and Ca2+-dependent protein kinase C from human placenta catalyzes the Mg-ATP-dependent phosphorylation of serine residues of purified rabbit muscle actin. Two tryptic [32P]-phosphopeptides were found on HPLC separation. Confirming the previous report by Machicao and Wieland [(1985) Curr. Top. Cell. Regul. 27, 95-105], actin is phosphorylated at serine residues by human placental membranes, and this is stimulated by insulin. In the absence of insulin trypsin treatment yielded eight [32P]phosphopeptides, two of which coincided with the ones due to protein kinase C. Insulin led to the appearance of three new [32P]phosphopeptides. These results suggest that insulin stimulates (a) serine protein kinase(s) which, like protein kinase C, is present in placental membranes.

Cellular compartmentation of Ehrlich ascites tumor cells

The digitonin method for the study of cellular compartmentation in mitochondrial and cytosolic fractions was applied to Ehrlich ascites tumor cells. The volume of mitochondrial and cytosolic water spaces are calculated to be 1.62 microliter/30 x 10(6) cells respectively, by the technique of 3H2O permeable and (14C)-sucrose impermeable spaces. The validity of the methods was tested by the distribution of cytosolic (lactate dehydrogenase) and mitochondrial (citrate synthase and glutamate dehydrogenase) marker enzymes. As occurs in normal hepatic cells, an asymmetric distribution of ATP and ADP was observed. The ATP/ADP ratio in the cytosolic fraction was 7 times higher than in the mitochondrial fraction.

Nitrogen movement between host and tumor in mice inoculated with Ehrlich ascitic tumor cells

Tumors function as a nitrogen trap, and they compete with the host for nitrogen compounds. In experiments with whole animals infected with Ehrlich ascitic tumor cells, the glutamine, glutamate, asparagine, and aspartate concentrations were determined for host plasma, ascitic liquid, and tumor cells, throughout the period of tumor growth. Concentration gradients of glutamine or asparagine were created from the host tissues towards the ascitic liquid. The countergradient step from ascitic liquid to tumor cells may be overcome by an active transport process with an apparent Km for glutamine of 3.1 X 10(-4) M. On the other hand, a reverse flux of glutamate and aspartate was seen to take place from cells to plasma. In vitro incubations of tumor cells with near physiological concentrations of glutamine, or asparagine plus glucose, confirmed the host-to-tumor nitrogen movement previously deduced from the relative amino acid concentrations in plasma, ascitic liquid, and tumor cells. The ammonemia detected in tumor-bearing mice at the terminal stage could result from the hydrolysis of glutamine, which was rapidly metabolized by the tumor cells.

Ammonium ions enhance the flow through the pyruvate dehydrogenase in Ehrlich ascites tumor cells

The flow through pyruvate dehydrogenase was assayed in glycolysing cells by the evolution of 14CO2 from [1-14C] pyruvate. Parallel incubations were carried out in high bicarbonate buffer (25 mM) and in bicarbonate-free buffer. The activation of the complex by NH+4 was only observed in high bicarbonate buffer, because the dilution of labelled CO2 in the presence of an excess of bicarbonate enables the quantitative determination of labelled CO2 evolved from pyruvate in the decarboxylase step. In the bicarbonate-free buffer the activation of the complex was not observed, because the 14CO2 evolved from pyruvate was consumed by biosynthetic processes inside the cell. On the contrary in isolated hepatocytes the NH+4 activation of the pyruvate dehydrogenase was observed in both buffers. In Ehrlich ascites cells, in common with other mammalian tissues, pyruvate dehydrogenase activity was found to be inversely correlated to the intramitochondrial ATP/ADP ratio.

Metabolism of acetaldehyde and custers effect in the yeast

Brettanomyces abstinens growing on different initial glucose concentrations showed an anaerobic inhibition of fermentation. This Custers effect decreased as the initial glucose concentration in the medium increased. Two aldehyde dehydrogenases, one NAD+-linked and the other NADP+-linked were observed. The results suggest that the NAD+-linked enzyme is involved in the production of acetic acid and is repressed by glucose. The NADP+-linked enzyme seems to be a constitutive enzyme. Acetyl-CoA synthetase activity also was not greatly affected by the growth conditions. The results support the earlier hypothesis that the Clusters effect in Brettanomyces is provoked by the reduction of NAD+ in the conversion of acetaldehyde to acetic acid.