Abstract
Conidia pregerminated with fructose were incubated with increasing amounts of 14c-labelled sorbose and their radioactivity was measured by millipore filter technique. Sorbose absorption followed Michaelis-Menten kinetics, with [image]m =5.1 [plus or minus] 1.2 m[image] and [image]max = 0.16 [plus or minus] 0.05 mg sorbose/mg dry weight/min. It was inhibited slightly by addition of fructose, and strongly by addition of glucose; the exact type of inhibition was dependent on concentration. Ungerminated conidia or pregerminated conidia were incubated with 14C-labelled sorbose. A base level of sorbose absorption by ungerminated conidia, and an increasing absorption with increasing pregermination time of the conidia was found; the absorption was proportional to germ-tube length. Supernatants of ungerminated or gernated conidia were checked for sugars related to sorbose by enzymatic and bioassays. The results were negative. Radiopaper chromatography 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. Nearly 90% of the fructose but barely 7% of sorbose taken up is transformed into a water insoluble form after 60 min. incubation of the conidia. Conidia incubated with labelled sorbose were treated with unlabelled sorbose or sodium azide. The accumulated labelled sorbose was driven out by both treatments (with sorbose nearly 65%, with sodium azide nearly 80% after 60 min.). The data support the hypothesis proposed earlier that sorbose is absorbed by conidia of Neurospora crassa by means of active transport mediated by a inducible enzymatic system of the permease type.