V O L U M E 2 7 , NO. 7, J U L Y 1 9 5 5 = 0.02 ml. of a 0.2% solution = 0.04 mg. Substit,uting:
VL
'I
=
81 1 1 X - X - = 3.01 micromoles/minute/mg. 22 4 30 0 04
Table I11 summarizes the specific activities as calculated for each of the methods above together with the standard deviation to be expected for each met,hod of assay. The variability found for each method as calculated from the ratio of the standard deviations t o the mean specific activit,y is wit'hin a few per cent in every instance. STABILITY O F CHOLISEBTER.4SE DISKS UPON STOR.4GE. The stabilit,y of the disks prepared in the various media was studied by determining the activity of freshly made disks by one cr more of the methods previously described; t'hen st,oring similar diska under the conditions and for the time noted in Table IV followed by a det,ermination of their activity. Inspection of Table I T T shorn that optimal retention of initial activity i.5 achieved by storing in the cold over a desiccant. Disks decrease t o 62% of t,heir initial act,ivity in 22 days when exposed t o an atmosphere of 100% humidity a t room temperature. The effect of the stabilization inediuni is shown by a coniparison of the disks prepared in the bovine albumin n-ith those prepared in the gelatin medium and stored under identical conditions. The disks prepared in the 47, bovine albumin had lost virtually no activity in 6 months, whereas those prepared in the gelatin medium had lost one fourt,h of t,heir initial activity. DISCUSSION
The conditions used in the four methods for measuring the cholinesterase activity of the disks differ in salt concentration, presence of protein stabilizer, substrate concentration, and period of time used in measurement,. Because these factors all affect the stability and activity of the enzyme, it is to be expected that the specific cholinesterase act,ivit)- of a given preparation, meaeured by different procedures but expressed in the same units, will vary from one method to the nest. However, the variability of 2 to 3% found for the activity of the disks by any of the procedures described here permits their use in calibrating the activity measured bj- one met,hod in terms of another, provided that a given set of conditions is consistently followed. I n addition, the disks may serve to point out sources of experimental variations
1083 where these are significantly greater than the 2 to 3% variation attributable to the disks themselves. As the degree of purity of the original enzyme preparation ultimately determines the activity of the disks, preparations other than the one used in this study will yield different specific activities. Standardization of different techniques may nevertheless be achieved with any such preparation. Under the conditions specified in this report the titrimetric procedure gave the highest specific activity, followed in turn by the colorimetric, manometric, and electrometric procedures. The last named method has the advantage of requiring the fewest reagents, and permitting inany samples t o be measured with a minimum of manual operations, thus minimizing the tediousnem involved in the careful performance of the titrimetric procedure when the latter is performed manually. The colorimetric method, while requiring more reagents than the other procedures, is the fastest, and compares favorably in precision and sensitivity, with any of t,he methods studied. If eluates of filter paper standards are used for inhibition studies a t inhibitor concentrations yielding a lo^ level of cholinesterase activity, t'hen the colorimetric method offers t'he additional advantage of determining the amount of acetylcholine remaining after exposure to the residual enzyme in the eluate, rather than the small amount of acetic acid formed in excess of the blank determination. .4CKNOWLEDGMENT
The authors wish to thank Bernard J. Jandorf for helpful criticism on the manuscript. LITERATURE CITED
(1) Ainsmorth. A I . , Davies, D. R., and Rutland, J. P., to be published. ( 2 ) Ammon, R , Arch. g e s P h y s i o l (Pflugers). 233, 486-91 (1933). (3) Augustinsson, K B , and Heimburger, G , d c t a Physiol Scand., 30 (41.45-54 11953). . ~~~, (1) Berry;
