By Gould R.F. (ed.)
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Let us assume that an acetylcholine molecule covers a surface area of about 30-50 A . The amount of surface covered by acetylcholine metabolized in the squid axon per square centimeter surface per millisecond would be of an order 14 12 2 36 III. FEATURES OF ACETYLCHOLINESTERASE of magnitude from 5 Χ 10 to 5 χ ΙΟ μ . This is an extremely small fraction of the total surface. The figures are sufficiently low to account for the relatively small amount of heat observed, even assuming the actual heat production to be several times as high as that used in Hill's calculations, in view of the recent data on Maja nerve.
He used, how ever, the material without purification or fractionation. A significant progress in the characterization of acetylcholine-splitting enzymes resulted from the observations of Alles and Hawes ( 1 9 4 0 ) . These investigators found that the esterase of red blood cells differs markedly from that in the serum: the enzyme of xed blood cells has a well-defined optimum of substrate concentration. Excess of substrate decreases the activity of the enzyme. This is in sharp contrast to the activity of serum esterase which has no well-defined optimum of substrate concentration but shows the usual Michaelis-Menten type of curve of activity substrate concentration relationship.
Velocity in absence, v' = in presence of inhibitor, expressed in microliters of C 0 evolved in 3 0 min. Relative inhibitor concentration (Rel. [/]), 1 = 6 X 1 0 ~ M . ( · ) without, ( O ) with incubation, ( O ) values obtained with different substrate concentrations. Kj = 6 . 1 Χ 1 0 " . 2 7 8 strate and enzyme and inhibitor, respectively. The intercept of the straight line on the ordinate (ν/ν') is 1. K and Kj can be calculated from the slope of the line, that is K / K ( [ S ] + K ). Figures 12 and 13 show the data obtained in applying this method of analysis to the action of eserine and Prostigmine.