Inhibition of amino acid transport and enhancement of tryptophan binding in a water mold by a range of natural and synthetic cytokinins

Abstract

Approximately 30 purine compounds were examined for their influence on the energy-linked amino acid transport systems of Achlya (a freshwater mold). Most of them were inhibitory, and their effectiveness was related to (a) the presence of a substituent group at position 6 of the purine ring, (b) the length of hydrocarbon chain as substituent, and (c) the type of linkage between the substituent group and the heterocyclic ring. The most effective linkage group was SH followed by an NH2 group, and an oxygen bridge which was the least effective. Inhibition was immediate (suggesting an interaction with the cell membrane) and pH and temperature dependent. Only tryptophan uptake was anomalous, for while its transport was inhibited, its binding to a cell wall – membrane proteoglycan was stimulated severalfold. From kinetic studies, we deduced that the analogues and tryptophan were interacting with the same cell entity. Nucleoside forms of the analogues did not inhibit amino acid transport; this was understandable, since nucleosides are transported by an energy-linked system whereas nucleic acid bases are not. The purine analogues themselves were taken up very poorly by this organism.