부산대학교 도서관

1. Variation in activity of fly head cholinesterase (ChE) was measured titrimetrically at 25.0 degrees C. with ACh.Br 0.015 Mas substrate, as a function of the pH of the assay medium over the range from pH 4.0 to 10.0. Ground tissue obtained from Musca domesticaL. was suspended at a concentration of one head per ml. in three media: (1) buffer of composition NaCl, 26.30 gm.; KH2PO4, 3.85 gm.; NaOH, 1.00 gm.; H2O, to one liter; (2) 30 per cent glycerol; (3) de-ionized water.2. Enzymic activity was greater in buffer than in the other media. The pH optimum was definitely on the alkaline side, being at least as high as pH 8.0 and probably as high as 9.0. In glycerol or water suspensions, enzymic activity changed little between pH 6.0 and 9.0.3. Some permanent inactivation of the enzyme was observed in half-hour exposures at high and low pH values. This effect was measured over the pH range from 3.0 to 12.0. Between pH 5.0 and 10.5, the degree of inactivation was moderate and essentially complete within 30 minutes. The time course of the process was followed at pH 4.0 for intervals from 15 minutes to two hours, and appeared to involve a rapid phase during the initial 30 minutes and a slower phase thereafter. Both phases had the characteristics of a first order reaction. Inactivation of ChE resulting from exposure to low or high pH was not reversed during subsequent incubation of the sample at pH 8.0 for as long as 18 hours.4. Correction of the pH-activity curve to allow for changes in the relative amounts of enzyme that result from permanent inactivation requires only minor alterations, since the effect of pH on reaction rate makes itself felt within pH limits where the degree of permanent inactivation is slight.