Heme regulation of HeLa cell transferrin receptor number

The number of diferic transferrin receptors on HeLa cells decreases when cells are grown in iron-supplemented media. The experiments reported here suggest that heme is the iron-containing compound which serves as the signal for receptor number regulation. When HeLa cells were grown in the presence of hemin, transferrin receptor number decreased to a greater degree than when cells were grown in equivalent amounts of iron supplied as ferric ammonium citrate. Incubation of cells in conditions which increased cellular heme content resulted in a decrease in cellular transferrin receptors. Incubating cells with 5-aminolevulinic acid (thus bypassing the rate-limiting step in heme biosynthesis, 5-aminolevulinic acid synthase) led to a decrease in transferrin receptor number. Incubation of cells with an inhibitor of heme oxygenase, Sn-protoporphyrin IX, also led to a decrease in transferrin receptor number. When cellular heme content was decreased by inhibiting heme synthesis with succinylacetone (an inhibitor of 5-aminolevulinic acid dehydratase), or by depriving cells of iron with deferoxamine, an increase in HeLa cell transferrin receptor number was seen. When HeLa cells were incubated with inducers of heme oxygenase (CoCl2, SnCl2, Co-protoporphyrin IX), transferrin receptor number also increased. The effects of all compounds which alter transferrin receptor number were dependent on the concentration of the supplement, as well as the duration of the supplementation. These experiments suggest that intracellular heme content may be an important signal controlling transferrin receptor number.     

The chemistry of vitamin B12. The coordination of biologically important molecules

The following equilibrium constants (given as logK in units of m⁻¹) were determined for the substitution of co-ordinated H₂O in aquocobalamin by glycine (bound through N) 5.8, cysteine (bound through S) 6.0 or 8.3, depending on the value chosen for the pK of the thiol group, and phenolate 2.9. The spectrum of the phenolate cobalamin shows an additional intense absorption band at 468nm with a molar extinction coefficient of 1.1×10⁴, which is assigned to a charge transfer from the phenolate to the cobalt ion. Equilibrium constants have also been determined for the equilibria between adenylcobamide cyanide and CN⁻, HO⁻ and H⁺, which show that the adenine is more easily displaced by CN⁻ and HO⁻ than is 5,6-dimethylbenziminazole in vitamin B₁₂, but can be protonated by acid while still remaining co-ordinated to the cobalt. It is shown that in the binding of corrinoids to proteins and polypeptides the formation of hydrogen bonds is far more important than co-ordination by the metal.     

Dynamic response of the peripheral chemoreflex loop to changes in end-tidal O2.

We studied the peripheral ventilatory response dynamics to changes in end-tidal O2 tension (PETO2) in 13 cats anesthetized with alpha-chloralose-urethan. The arterial O2 tension in the medulla oblongata was kept constant using the technique of artificial perfusion of the brain stem. At constant end-tidal CO2 tension, 72 ventilatory on-responses due to stepwise changes in PETO2 from hyperoxia (45-55 kPa) to hypoxia (4.7-9.0 kPa) and 62 ventilatory off-responses due to changes from hypoxia to hyperoxia were assessed. We fitted two exponential functions with the same time delay to the breath-by-breath ventilation and found a fast and a slow component in 85% of the ventilatory on-responses and in 76% of the off-responses. The time constant of the fast component of the ventilatory on-response was 1.6 +/- 1.5 (SD) s, and that of the off-response was 2.4 +/- 1.3 s; the gain of the on-response was smaller than that of the off-response (P = 0.020). For the slow component, the time constant of the on-response (72.6 +/- 36.4 s) was larger (P = 0.028) than that of the off-response (43.7 +/- 28.3 s), whereas the gain of the on-response exceeded that of the off-response (P = 0.031). We conclude that the ventilatory response of the peripheral chemoreflex loop to stepwise changes in PETO2 contains a fast and a slow component.     

Reductive biotransformations of organic compounds by cells or enzymes of yeast.

Saccharomyces cerevisiae catalyses the asymmetric reductive biotransformation of a variety of compounds containing a carbonyl group or carbon-carbon double bond. Oxidoreductases participating in these reactions which have commercial potential in biotransformation processes are likely to have relatively broad substrate specificity. Important carbonyl reductases falling into this category include YADH- and yeast NADP-dependent beta-ketoester reductases. The enoyl reductase component of the FAS complex may have a role in asymmetric yeast reduction of carbon-carbon double bonds of unnatural substrates. Other nicotinamide-requiring oxidoreductases of yeast are also surveyed to rationalize observed biotransformations of whole yeast cells in terms of specific enzymes. Genetic and protein engineering may enable enzymes to be tailored to accept new substrates. A greater understanding of the enzymes and reactions involved will facilitate further optimization and exploitation of these catalytic systems in industrial processes.