Calorimetry of cholinesterase activity in serum

Cholinesterase activity in human serum was measured calorimetrically and compared to values obtained with the spectrophotometric method of Rappaport et al. [Clin. Chim. Acta 4, 227 (1959)]. The response of the calorimeter upon mixing serum with acetylcholine corresponded to the cholinesterase activity of the serum, the thermogram peak height being linearly related to enzyme activity at serum dilution volumes at which interference heat was negligible. The interference heat contributed by the dilution mixing characteristics of serum, studied separately, was found to be similar for heat-denatured, aged, or lyophilized samples. In all cases, heat of mixing was negligible when 1 ml of serum in buffer (2/3 by vol) was mixed with 1 ml of buffer. When this amount of serum was assayed with acetylcholine, cholinesterase activities of 2 and 3 kU/liter were recorded as thermograms with peak heights of 2.3 and 3.5 cm, respectively.