Cooperativity of the concanavalin A inhibition of bovine milk fat globule membrane 5-nucleotidase
1980
Snow, L.D. | Doss, R.C. | Carraway, K.L. (Oklahoma State Univ., Stillwater (USA). Dept. of Biochemistry)
5''-Nucleotidase (5-ribonucleotide phosphohydrolase, EC 3.1.3.5) of bovine milk fat globules can be solubilized by deoxycholate from either isolated globule membranes or washed cream. The solubilized and membrane-bound enzymes exhibit similar K(, m) values and are inhibited by concanavalin A by an apparent noncompetitive process. The soluble enzyme shows positive cooperativity for the inhibition (Hill coefficient of 2) at 37 deg C, but the membrane enzyme exhibits essentially no cooperative effect. At lower temperatures (5 or 20 deg C) the cooperative effect in the inhibition of the soluble enzyme is lost. Colchicine and cytochalasin D failed to induce cooperativity of the concanavalin A inhibition of the membrane enzyme, but induction of cooperativity occurred when membranes were extracted with glycine/EDTA/mercapto-ethanol, releasing a major protein component with a polypeptide molecular weight of 155 000. We suggest that the interaction of this component with the membrane imposes restraints on the behaviour of the nucleotidase which are reflected in the cooperativity of the inhibition of the enzyme by concanavalin A
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