Cholecystokinin-converting enzymes in brain.

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Abstract
Crude extracts of porcine cerebral cortical tissue convert cholecystokinin (CCK) to its COOH-terminal fragments, the dodecapeptide (CCK-12) and the octapeptide (CCK-8). The Sephadex G-75 void volume eluate of the crude extract cleaves the arginine-isoleucine bond and effects conversion only to CCK-12; the Sephadex G-50 void volume eluate of the same extract cleaves the arginine-aspartate bond as well, so that CCK-12 and CCK-8 are end products. There are at least 2 enzymes; the 1 involved in the conversion to CCK-12 is of larger molecular radius than the other. The Km for the cleavage of CCK at the arginine-isoleucine bond by the Sephadex G-75 void volume eluate enzyme is 1.1 .times. 10-6 M; the Km for trypsin cleavage of the same bond is 4.7 .times. 10-6M. The lower Vmax for the brain enzyme (1.5 .times. 10-11 mol/min per g of extract) compared with trypsin (66 .times. 10-11 mol/min per g of trypsin) simply reflects the lesser degree of purity of the brain extract than of the highly purified trypsin.

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