A

(Pancreatic icosapeptide)

FIGURE 7-20 Biosynthetic pathway for human pancreatic polypeptide. The prepropancreatic polypeptide peptide has 66 amino acids preceded by 29 amino acid residues. A sequence of three basic amino acids (G, K, R) is processed by three enzymes: (i) a paired basic residue endopeptidase recognizing Lys-Arg (K,R), (ii) a carboxypeptidase B-like enzyme that removes the C-terminal arginine and lysine successively, and (iii) an amidating enzyme system that oxidatively removes the glycine (G) to yield C02 and the preceding carboxyamidated amino acid (Tyr or Y). [Modified with permission from Steiner, D. F., Bell, G. I., Tager, H. S„ and Rubenstein, A. H. (1995). Chemistry and biosynthesis of the islet hormones: Insulin, islet amyloid polypeptide (amylin), glucagon, somatostatin and pancreatic polypeptide. In "Endocrinology" (L. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaux, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd edi., Vol. 2, Chapter 76, pp. 1296-1328. W. B. Saunders Co., Philadelphia, PA.]

K+ channel and inward Ca2+ channel, is described in Figure 7-23. In this model, the intracellular enzyme glucokinase, which converts glucose to glucose 6-phosphate, is believed to play a key role in determining the rate of metabolism of glucose, which will ultimately lead to changes in the intracellular ratio of [ATP]/ [ADP]. When this ratio is elevated, the K+ channel is inhibited, resulting in membrane depolarization, with concomitant opening of the Ca2+ channel so as to in

TABLE 7-8 Modulators of Insulin and Glucagon Secretion
0 0

Post a comment