Effect of Cycloheximide on l-Leucine Transport by Penicillium chrysogenum: Involvement of Calcium

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RESUMO

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. Ca2+ reverses the inhibition of transport caused by phosphate plus cycloheximide. Ca2+ did not relieve the inhibition of protein synthesis. Cycloheximide promotes a continual uptake of 45Ca2+ by the mycelium. The cumulative results suggest that (i) membrane-bound Ca2+ is involved in amino acid transport, (ii) cycloheximide labilizes the membrane-bound Ca2+, and (iii) phosphate forms a complex with Ca2+ 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.

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