Abstract
It has been shown by Anson and Mirsky 1 in a long series of studies that the denaturation of many proteins is a reversible process. The isolation of crystalline trypsin 2 and pepsin 3 has furnished proof that at least some enzymes are protein compounds. In them the denaturative reaction is paralleled by loss of enzymatic activity and conversely, as the denatured protein is changed back to native protein this activity is regained. 4
Krueger 5 found that the thermal inactivation of phage follows the course of a unimolecular reaction and that the critical thermal increment for inactivation is about 100,000. Since critical thermal increments of this order of magnitude are uniquely characteristic of protein-denaturation he concluded that the inactivation of phage by heat involved protein-denaturation as a significant reaction. Additional experimental support for this concept is available in other observations:
1. Vedder 6 has noted that the loss of activity on heating dried phage is much less than with phage in aqueous suspension; dried proteins are typically more thermostable than proteins in solution.
2. While immunological experiments are not theoretically conclusive they are most readily interpreted by considering phage to be a protein capable of functioning as an antigen. 7
3. Purified phage gives the usual chemical tests for proteins and on elementary analysis has been found to contain carbon, hydrogen, nitrogen and phosphorus 8 , 9 in proportions empirically compatible with a protein structure.
Taken together these data suggest that phage is composed of one or more proteins and that its lytic activity is intimately linked with maintenance of the native protein structure. This would be entirely in keeping with the view of some workers that phage is an enzyme and it must be admitted that certain analogies exist between the properties of phage and those of enzymes.
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