Polyamine transport in Neurospora crassa.

Polyamine transport in Neurospora crassa is concentrative and energy dependent in a dilute buffer. The saturable systems governing the uptake of putrescine (Km = 0.6 mM), spermidine (Km = ca. 0.24 mM), and spermine (Km = 0.07 mM) share components, as indicated by mutual inhibition among the polyamines. In addition, nonsaturable components prevail for putrescine and spermidine, particularly the former. Radiolabeled substrates, once in the cell, are released only slowly, even if unlabeled polyamines are included in the incubation medium. Permeabilization of cells with n-butanol leads to partial release of internalized 14C-polyamines, and the remainder is almost wholly exchangeable with added, unlabeled polyamine. Polyamine uptake was inhibited by the polyamines themselves and by a polyamine analog, methylglyoxal bisguanylhydrazone, but only weakly and incompletely by the basic amino acids arginine and ornithine. Uptake of putrescine and spermidine was inhibited by monovalent cations, Ca2+, and certain other components of the growth medium. As a result, uptake from the growth medium was very slow and largely by way of the nonsaturable uptake mechanism.