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Adenylate cyclase in the rat pancreas properties and stimulation by hormones

1972; Elsevier BV; Volume: 274; Issue: 1 Linguagem: Inglês

10.1016/0005-2736(72)90294-5

1879-2642

W.J. Rutten, J.J.H.H.M. De Pont, S.L. Bonting,

Aldose Reductase and Taurine

1. In view of evidence for a role of cyclic AMP in the secretin-stimulated water and electrolyte secretion and the pancreozymin-stimulated enzyme secretion by the pancreas, the occurrence of adenylate cyclase in the pancreas has been investigated. 2. The presence of the enzyme in a particulate fraction of rat pancreas has been demonstrated. Basal activity is very low under standard conditions (less than 3 pmoles/min per mg protein), but 10 mM NaF stimulates the enzyme to 15–40 pmoles/min per mg protein. 3. Properties of the enzyme have been determined in the presence of 10 mM NaF. The apparent Km for ATP, derived from a Lineweaver-Burk plot is 0.3 mM. At ATP concentrations above 1.5 mM there is substrate inhibition. The optimal pH is 7.4. The enzyme requires Mg2+ for its activity, concentrations of 5 mM and higher giving maximal activity. Ca2+ in concentrations above 0.1 mM inhibits the enzyme. 4. The enzyme is stimulated by a synthetic secretin preparation (half maximally activating concentration 1.5·10−8 M) as well as by a purified pancreozymin preparation (half maximally activating concentration 1.5·10−6 M). The latter activation is not due to a contamination of the pancreozymin preparation by secretin. The secretin-stimulated enzyme has the same optimal pH of 7.4 and shows the same Ca2+ inhibition as the NaF-stimulated enzyme. 5. The enzyme is not stimulated by adrenalin and its analogues, by acetylcholine and its analogues, and by the hormones glucagon and gastrin (synthetic). 6. Specific enzyme activity (per mg protein) in enzyme preparations obtained from rats with free access to food and stimulated in vivo by secretin and pancreozymin is twice as high as that in enzyme preparations from starved animals. 7. The physiological significance of these findings is discussed.