Aktive Aufnahme von Zuckern durch Zellen von Neurospora crassa unter Beteiligung eines enzymatischen Systems mit Permease-Eigenschaften II

Abstract

Sorbose-transport in Neurospora has been investigated further. Methods and results were as follows: Conidia pregerminated with fructose were incubated with increasing amounts of 14C-labelled sorbose, their radioactivity was measured by millipore-filter technique. Sorbose uptake followed Michaelis-Menten kinetics, with Km = 5.1 ± 1.2 mM and Vmax=0.16 ± 0.05 mg sorbose/mg dry weight/minute. It was inhibited slightly by addition of fructose, and strongly by addition of glucose; the exact type of inhibition was concentration dependent. Ungerminated conidia or pregerminated conidia were incubated with 14C-labelled sorbose. A base level of sorbose uptake by ungerminated conidia, and an increasing uptake with increasing pregermination time of the conidia was found; the uptake was proportional to germ-tube length. Supernatants of ungerminated or germinated conidia were checked for sugars related to sorbose by enzymatic and bio-assays. The results were negative. Radiopaperchromatography of aqueous extracts of incubated conidia revealed that sorbose as opposed to fructose is not phosphorylated during uptake, but accumulated as the pure sugar inside the cells. Ca. 90% of the fructose but barely 7% of sorbose taken up is transformed into a water insoluble form after 60 minutes incubation of the conidia. Conidia incubated with labelled sorbose were treated with unlabelled sorbose or Na-azide. The accumulated labelled sorbose was driven out by both treatments (with sorbose ca. 65% with Naazide ca. 80% after 60 minutes). The data support the hypothesis proposed earlier1 that sorbose is taken up into conidia of Neurospora crassa by means of active transport, mediated by an inducible enzymatic system of the permease-type.