Small fragments from the A subunit of cholera toxin capable of activating adenylate cyclase.

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Abstract
Exposure of cholera toxin to membrane particles prepared from [mouse] sarcoma 180 cells gives rise to a variety of fragments which are capable of activating adenylate cyclase [ATP:pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. A major component of these fragments has an apparent MW in the 8000-10,000 range. The smallest stimulatory fragments has a MW of approximately 1400. The small size of the fragments is confirmed by Sephadex gel filtration, in the presence of sodium dodecyl sulfate or formic acid. These fragments are produced from holotoxin or its A subunit by protease(s) found in sarcoma membrane particles. Production of fragments appears optimal in 40-60 min at 30.degree. and pH 7, and is prevented by protease inhibitors. The ability of the small fragments to activate adenylate cyclase is reversed by [horse] by anti-holotoxin, but not [horse] anticholeragenoid, antibodies. These fragments require NAD for the activation of adenylate cyclase and are fully active after heating at 90.degree. for 5 min (pH 7).