Description:
<jats:p>In an effort to explain sorbose-toxicity in Neurospora crassa <jats:sup>1,2</jats:sup>, two alternatives were considered: inhibition by sorbose of enzymes responsible for fructose-phosphorylation (enzyme-inhibition), or competition of sorbose with fructose for a common uptake mechanism (permease-competition).</jats:p>
<jats:p>By means of combined enzymatic optic tests alternative one (enzyme-inhibition) was ruled out for the reaction fructose → fructose-6-phosphate. It may still apply however to a reaction further down in fructose catabolism.</jats:p>
<jats:p>To check alternative two (permease-competition), studies on the uptake of <jats:sup>14</jats:sup>C-labelled sugars by pregerminated conidia were carried out, measuring radioactivity of intact conidia (membranefiltertechnique) or of the supernatant of boiled or crushed conidia with increasing incubation times.</jats:p>
<jats:p>It was found that rates of uptake of sorbose, fructose and glucose differ from one another. The uptake of these sugars is strongly decreased in the cold and even more by the addition of Na-azide. The uptake is thus energy-dependent. Sorbose is accumulated unchanged inside the cells up to concentrations &gt;600 times that offered in the incubation medium. Sorbose uptake is thus active. Sorbose is taken up quickly after pregermination of conidia with fructose, but slowly after pregermination of conidia with glucose. The involvement of an inducible enzyme system of the permease type in sorbose transport is thus likely.</jats:p>