Uptake, intracellular binding, and excretion of polyamines during growth of Neurospora crassa

Rowland H Davis,Janet L Ristow

doi:10.1016/0003-9861(89)90281-6

Rowland H Davis, Janet L Ristow

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In Neurospora crassa mycelia, the amounts of the main polyamines, putrescine and spermidine, are approximately 0.8 and 18 nmol/mg, dry weight. We wished to know what determines these pool sizes. In the growth medium, externally added polyamines enter cells largely by a nonsaturable, diffusional system. In a mutant unable to synthesize polyamines, internal and external spermidine appear to equilibrate across the cell membrane during growth. However, this was true only after an intracellular “sink,” with a capacity equal to the amount of spermidine found in wild-type cells, had been saturated. We speculate that internal anionic binding sites, detectable in permeabilized cells, sequester virtually all of the spermidine normally found in exponentially growing N. crassa. Further evidence for this view was that in mature, stationary cultures, excess spermidine is excreted. Putrescine is also excreted if its concentration in the cell is abnormally high. The control of pool size by intracellular binding and excretion may be an advantage in this pathway, because feedback inhibition does not prevail, enzyme regulation is by comparison slow, and excessive polyamines are toxic.

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In Neurospora crassa mycelia, the amounts of the main polyamines, putrescine and spermidine, are approximately 0.8 and 18 nmol/mg, dry weight
The control of pool size by intracellular binding and excretion may be an advantage in this pathway, because feedback inhibition does not prevail, enzyme regulation is by comparison slow, and excessive polyamines are toxic. ?. 1989 Academic Press
We have shown that in the growth medium, the saturable polyamine uptake systems seen in dilute buffer are virtually inactive, and that entry of polyamines is largely diffusional

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In Neurospora crassa mycelia, the amounts of the main polyamines, putrescine and spermidine, are approximately 0.8 and 18 nmol/mg, dry weight. We speculate that internal anionic binding sites, detectable in permeabilized cells, sequester virtually all of the spermidine normally found in exponentially growing N. crassa Further evidence for this view was that in mature, stationary cultures, excess spermidine is excreted. The polyamines have several peculiarities not shared by most biosynthetic intermediates They are sequestered in cells (l), and it is likely that they bind to anionic cell constituents such as ribosomes, DNA, polyphosphates, and phospholipids (l-3). Higher organisms, spermine, are “deadend” products: they are needed in few further biochemical reactions, and even catabolism of the polyamines may be restricted Under these circumstances, it is odd that efficient feedback inhibition of ornithine decarboxylase, a key biosynthetic enzyme, has never evolved in any eucaryote. This is unusual because high levels of polyamines may be toxic, even in an organism like Neurospwa crassa, where little polyamine turnover takes place [1,4,5]

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