Pepsin ketene

Careful acetylation of pepsin with ketene has yielded preparations with no free amino groups, but with full activity. More extensive acetylation inactivates the enzyme. Tyrosine side chains appear to be somewhat more important to the enzyme, but some activity remains after partial iodination, and treatment with nitrous acid leaves 50 per cent of the activity. 

The reviewer has been surprised at the high degree of homogeneity of certain reaction mixtures. When pepsin was treated with ketene (9) or iodine (7) under mild and controlled conditions the bulk of the reaction mixture was relatively uniform, as shown by fractionation procedmres designed to detect the presence of products with extremely different properties. This result may be interpreted as indicating that most of the groups of the type which react first, i.e., at the highest rate, are saturated before those of the next type have reacted appreciably. 

The action of ketene on solutions of crystalline pepsin has been studied in some detail (4). It was shown that at pH 5.5 the amino groups of the protein were the first to be acetylated. The loss of the Van Slyke amino nitrogen was just equivalent to the increase in number of acetyl groups. This derivative was crystallized and it had the same enzyme... 

All the amino groups of tobacco mosaic virus are not acetylated even after extensive treatment with ketene (Miller and Stanley, 92). Pappen-heimer s (173) experiments on diphtheria tmdn also indicate the presence of amino groups of differing reactivity. In the work on pepsin (4) it was concluded that all the amino groups were covered, for the final amino nitrogen value differed from the blank value by less than the experimental error. 

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