. Analysis of development. Embryology; Embryology. Cellular Metabolism 81 carry an inactive reserve of enzymes to be used only in periods of unusual stress. This is self-evident in tissues like muscle, in which there is a clear-cut difference between rest- ing and activity metabolism; but it is prob- ably true for many other tissues also. Cells of embryonic tissues, however, may well operate with virtually no safety factors in enzyme concentration. Such cells presumably are never "resting" in the sense that adult muscle fibers or gland cells rest, and accord- ingly enzyme systems mig
. Analysis of development. Embryology; Embryology. Cellular Metabolism 81 carry an inactive reserve of enzymes to be used only in periods of unusual stress. This is self-evident in tissues like muscle, in which there is a clear-cut difference between rest- ing and activity metabolism; but it is prob- ably true for many other tissues also. Cells of embryonic tissues, however, may well operate with virtually no safety factors in enzyme concentration. Such cells presumably are never "resting" in the sense that adult muscle fibers or gland cells rest, and accord- ingly enzyme systems might be expected to function continuously at near maximum activity. A second element of uncertainty in the homogenate technique is the influence on activity of association of enzyme with struc- tural components of the cell. Although grind- ing tissue may destroy all the coarser ele- ments of cell structure, it is well known, as we have pointed out before, that different enzymes show varying affinities for particu- late matter in breis. Since enzyme activity must certainly depend in large part on pre- cise steric configurations, such as may be involved in the antigen-antibody situation (cf. Pauling, '48), it would not be surprising if a given number of enzyme molecules bound onto particles would exhibit a differ- ent activity from the same number of molecules in free solvition (Mazia and Blvim- enthal, '50). That such variations do occur is indicated by several studies in which different homog- enization media were used. In the case of muscle apyrase, for example, a water ho- mogenate has high activity but is insensitive to Ca**, whereas extraction with strong po- tassium chloride produces a soluble enzyme preparation of low activity which is strongly activated by Ca""* (Steinbach, '49). Similar effects have been noted with apyrase in the granules of chick embryo breis (Steinbach and Moog, '45). In neither case is there any assurance that only one enzyme is being dealt w
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