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Colchicine-binding activity in the homogenate fractions of the whole gastric epithelium and oxynein tissue has been studied to determine if tubulin is present in the bullfrog stomach. Most of the specific binding activity is found in the 100,000-g soluble fraction and is enriched in the oxynein tissue. Binding is proportional to protein concentration, linear up to 30 min, and half saturable at 16 microM colchicine concentration. Scatchard and direct linear plot analysis yields the following values for binding constants: Kd = 12 microM, Ka = 0.08 X 10(6) M-1, maximal binding capacity (Bmax) = 0.19 X 10(-6) M, and n = 84 pmol/mg protein. Fiftyfold excess lumicolchicine does not affect colchicine binding. The binding activity has been partially purified by the temperature-sensitive cycling technique. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of the product shows a major band corresponding to tubulin in the 55,000-dalton region. Tubulin in isolated oxyntic cells can be visualized by indirect immunofluorescence. These data and the observed increase in colchicine-binding activity in the burimamide- over the histamine-treated tissue, possibly due to changes in the levels of tubulin polymerization, suggest a likely involvement of the cytoskeleton in gastric secretion.

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