engleski

Heterogeneity of human serum cholinesterase revealed by thiocholine substrates

The activity of human serum cholinesterase (BChE) was measured with substrates acetylthiocholine (ATCh), propionylthiocholine (PTCh) and butyrylthiocholine (BTCh) in the presence and absence of specific reversible (BW286C51) and progressive inhibitors (VX, iso-OMPA and BNPP), and after heath inactivation of the enzyme. Kinetic parameters were calculated by a non-linear regression analysis using three equations describing models of substrate hydrolysis. The activities of the enzyme against all the three substrates deviated from the Michaelis-Menten kinetics in a very similar way. The activities fitted reasonably well the equation assuming the binding of an additional substrate to the allosteric site on the enzyme. According to the kinetic constants ATCh was shown to be a less favourable substrate than PTCh or BTCh. In following heat inactivation of the enzyme and inhibition by progressive inhibitors the substrates were completely interchangeable. However, when the activity was measured by ATCh in the presence of the reversible inhibitor a higher degree of inhibition was obtained than with PTCh and BTCh. After the polyacrylamide homogenous gel electrophoresis of the serum the cholinesterase molecular forms were visualised by the substrates and with all three substrates the same pattern of the molecular forms was obtained. Also, to develop cholinesterase bands of equal intensity a longer time and/or higher ATCh concentration was needed than of two other substrates.

cholinesterase; inhibition; molecular forms

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