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Biomedical subjects

D R Hunter

Publications and source records attributed to D R Hunter.

48 records · Page 3Linked to original sources
PubMed1973

Effect of weak acids on amino acid transport by Penicillium chrysogenum: evidence for a proton or charge gradient as the driving force.

A variety of weak acids at and below their pK(a) are potent inhibitors of transport in Penicillium chrysogenum. The effective compounds include sorbate, benzoate, and propionate (common antifungal agents), indoleacetate (a plant hormone), acetylsalicylate (aspirin), hexachlorophene, and a yellow pigment produced by the mycelia under nutrient-deficient conditions, as well as the classical uncouplers 2,4-dinitrophenol, p-nitrophenol, and azide. The results suggest that a proton gradient or charge gradient is involved in energizing membrane transport in P. chrysogenum. The unionized form of the weak acids could discharge the gradient by diffusing through the membrane and ionizing when they reach an interior compartment of higher pH. Experiments with 2,4-dinitrophenol and p-nitrophenol established that the ionized species are not absorbed by the mycelium to any great extent. The transport inhibitors also caused a decrease in cellular adenosine 5'-triphosphate (ATP) levels, but there was no constant correlation between inhibition of transport and suppression of cellular ATP. A decrease in aeration of the mycelial suspension had the same effect on transport and ATP levels as the addition of a weak organic acid. The effects on transport rates and ATP levels were reversible. The instantaneous inhibition of [(14)C]l-leucine transport by NH(4) (-) (and vice-versa) in nitrogen-starved mycelia at pH values of 7 or below can be explained by competition for a common energy-coupling system. The inhibition is not observed in carbon-starved mycelia in which the NH(4) (+) transport system is absent or inactive (but the general amino acid transport is fully active), or in iodoacetate-treated mycelia in which the NH(4) (+) transport system has been differentially inactivated. At pH values greater than 7.0, NH(3) and HPO(4) (2-) inhibit transport, presumably by discharging the membrane proton or charge gradient. Aniline counteracts the inhibitory effect of NH(3) and HPO(4) (2-) possibly by acting as a proton reservoir or buffer within the membrane.

Acids↗
PubMed1973

Effect of cycloheximide on L-leucine transport by Penicillium chrysogenum: involvement of calcium.

Cycloheximide (actidione) has an immediate inhibitory effect on amino acid transport by nitrogen-starved or carbon-starved mycelium suspended in phosphate buffer. High concentrations of phosphate alone are slightly inhibitory; cycloheximide appears to potentiate the effect of phosphate. Ca(2+) reverses the inhibition of transport caused by phosphate plus cycloheximide. Ca(2+) did not relieve the inhibition of protein synthesis. Cycloheximide promotes a continual uptake of (45)Ca(2+) by the mycelium. The cumulative results suggest that (i) membrane-bound Ca(2+) is involved in amino acid transport, (ii) cycloheximide labilizes the membrane-bound Ca(2+), and (iii) phosphate forms a complex with Ca(2+) making it unavailable for its role in transport. The effect of cycloheximide described above is observed within 1 to 2 min after addition of the antibiotic. This initial inhibition occurs more rapidly with 10(-3) M cycloheximide than with 10(-5) M cycloheximide. However, after a longer preincubation time, a curious inverse relationship between cycloheximide concentration and amino acid transport is observed. The mycelium incubated with 10(-5) M cycloheximide remains strongly inhibited (unless the antibiotic is washed away). The mycelium incubated with 10(-3) M cycloheximide recovers about 40% of the transport activity lost during the rapid initial phase. We have no obvious explanation for the inverse effect.

Buffers↗