Some factors controlling cell polarity in chick retinal pigment epithelial cells in clonal culture

In clonal culture differentiated chick retinal pigmented epithelial (RPE) cells form a monolayer which shows little or no cellular division. The cells usually rest on a basal and reticular lamina and are polarized with their apical surface towards the medium. The apical surface is characterized by apical protrusions, an extensive apical web of microfilaments and junctional complexes which join the apical-lateral borders. A PA/S positive material with a felt-like appearance from the serum component of the medium coats the surfaces of the tissue culture plates. A similar material is found on any membrane filter which has been exposed to medium containing serum. When such a filter brought in contact with the upper surfaces of the RPE cells, the apical surface characteristics are lost, the cells often accumulate Alcian Blue positive material between the cells and the filter and secrete a reticular and a basal lamina, i.e. they establish a second basal surface. Once this has occurred, the cells appear to either detach from the plate and reverse their polarity, or undergo division forming two cell layers. In the latter case new apical surfaces are created between the cell layers but the cells appear to join to form circular structures rather than sheets. These results suggest that contact with this felt-like material initiates formation of a basal surface. They further suggest that where the apical surface has been converted to a basal one the cell attempts to restore the apical surface either by separating from the plate and reversing its polarity or by creating circular structures and developing new apices oriented toward the center of the circle.     

Quantitative analysis of flux along the gluconeogenic, glycolytic and pentose phosphate pathways under reducing conditions in hepatocytes isolated from fed rats.

Hepatocytes were isolated from the livers of fed rats and incubated with a mixture of glucose (10 mM), ribose (1 mM), mannose (4 mM), glycerol (3 mM), acetate (1.25 mM), and ethanol (5 mM) with one substrate labelled with 14C in any given incubation. Incorporation of label into CO2, glucose, glycogen, lipid glycerol and fatty acids, acetate and C-1 of glucose was measured at 20 and 40 min after the start of the incubation. The data (about 48 measurements for each interval) were used in conjunction with a single-compartment model of the reactions of the gluconeogenic, glycolytic and pentose phosphate pathways and a simplified model of the relevant mitochondrial reactions. An improved method of computer analysis of the equations describing the flow of label through each carbon atom of each metabolite under steady-state conditions was used to compute values for the 34 independent flux parameters in this model. A good fit to the data was obtained, thereby permitting good estimates of most of the fluxes in the pathways under consideration. The data show that: net flux above the level of the triose phosphates is gluconeogenic; label in the hexose phosphates is fully equilibrated by the second 20 min interval; the triose phosphate isomerase step does not equilibrate label between the triose phosphates; substrate cycles are operating at the glucose-glucose 6-phosphate, fructose 6-phosphate-fructose 1,6-bisphosphate and phosphoenolpyruvate-pyruvate-oxaloacetate cycles; and, although net flux through the enzymes catalysing the non-oxidative steps of the pentose phosphate pathway is small, bidirectional fluxes are large.     

New Regulatory Mutation Affecting Some of the Tryptophan Genes in Pseudomonas putida

Three indole analogues, 5-methylindole, 5-fluoroindole, and 7-methylindole, and the tryptophan analogue 5-fluorotryptophan were found to inhibit the growth of wild-type Pseudomonas putida. Mutants resistant to these analogues were obtained. Some of the 5-fluoroindole- and 5-fluorotryptophan-resistant strains exhibit an abnormality in the regulation of certain trp genes. These strains excrete anthranilate when grown in minimal medium in the presence or absence of the inhibitor. In these strains, the trpA, B, and D gene products, the first, second, and fourth enzymes of the tryptophan pathway, are produced in 20-fold excess over the normal wild-type level. The other enzymes of the pathway are unaffected. Exogenous tryptophan is still able to repress the expression of the trpABD cluster somewhat. Similarity between the 5-fluoroindole- and 5-fluorotryptophan-resistant strains suggests that the former compound becomes effective through conversion to the latter. Repression and derepression experiments with two anthranilate-excreting, 5-fluoroindole-resistant strains showed coordinate variation of the affected enzymes. The locus conferring resistance and excretion is not linked by transduction to any of the trp genes.     

Tryptophan Synthetic Pathway and Its Regulation in Chromobacterium violaceum

Extracts of Chromobacterium violaceum catalyzed all of the reactions involved in synthesizing tryptophan from chorismic acid. Tryptophan auxotrophs which had lost any of these activities did not produce the characteristic purple pigment, violacein, when grown on a medium in which tryptophan was limiting. Gel filtration of extracts allowed us to estimate molecular weights for the tryptophan enzymes. All of the enzymes appeared to have molecular weights below 100,000. No enzymes were observed to occur in aggregates. The specific activities of the enzymes of the tryptophan pathway did not change when mutants were grown under conditions of limiting or excess tryptophan. The first enzyme in the pathway, anthranilate synthetase, was subject to feedback control by the end product, tryptophan. Tryptophan acted as a noncompetitive inhibitor with respect to glutamine, one of the substrates for anthranilate synthetase, and as a competitive inhibitor of the reaction when chorismate, the other substrate, was varied. The nonlinearity observed in the Lineweaver-Burk plot in the latter case suggests that there may be more than one chorismate-binding site on anthranilate synthetase.