1. The isolated protein cyst coat of Colpoda steinii reacted with [14C]glucose to bind 14C label in a reaction that was not an artifact of bacterial contamination or of adsorption. 2. The reaction was enzymically catalysed, had optimum pH7.0–7.4 and a temperature optimum of 36–38°C, was non-competitively and reversibly inhibited by 10μm-p-hydroxymercuribenzoate and was specific for d-glucose. It had Km8.2×10−6m. 3. With the exception of C-1, all the carbon atoms of glucose were bound to the same extent and, measured relative to C-2 or C-6, the binding of C-1 varied between 0.45 and 0.82. The lost C-1 was not accounted for as carbon dioxide. 4. On prolonged incubation a coat preparation reacted with 3.6% of its own weight of glucose. 5. The label was tightly bound, but after acid treatment a variable proportion was recovered as glucose and there was no evidence for the release of any other 14C-containing compound. 6. Even after dissolution of the coat protein, bound label was not removed by treatment with periodate or lead tetra-acetate.
