Abstract
The older definition of keratin based on the physical properties and behavior towards the common proteolytic enzymes of the tissue proteins has been modified by Block and Vickery. 1 They define keratin as “a protein which is resistant to digestion by pepsin and trypsin, which is insoluble in dilute acids and alkalies, in water and in organic solvents, and which on acid hydrolysis, yields such quantities of histidine, lysine, and arginine, that the molecular ratios of these amino acids are respectively approximately as 1:4:12. “However, proteins which do not yield approximately such quantities of the basic amino acids even though they fulfill the other criteria of the definition are not considered by Block 2 to be true keratins. Thus, for example, neurokeratin, which is insoluble in dilute acids and alkalies, not digested by pepsin and trypsin, etc., was considered not to be a true keratin because it yielded histidine, lysine, and arginine in the molecular ratios of 1:2:2.
Human skin (cornified epithelium from the sole of the foot and scales from patients with exfoliative dermatoses of various kinds) was obtained through the kindness of Dr. H. S. Burr of New Haven, Dr. H. B. Lewis of Ann Arbor, and Dr. G. M. McKee of New York City. The washed skin was extracted with hot alcohol, ether, and digested with pepsin-hydrochloric acid. Fourteen gm. of the dried skin, after hydrolysis, yielded 6.0% of arginine, 0.82% of histidine, 4.3% of lysine, and 3.4% of cystine. The basic amino acids were isolated by the silver precipitation method 3 and cystine was determined colorimetrically. 4 These results agree remarkably well with those reported by Eckstein 5 who used the Van Slyke nitrogen distribution method.
The molecular ratios of histidine:lysine:arginine in the samples of skin analyzed by Eckstein and by ourselves are of the order of 1:6:7 and, therefore, the samples of tissue analyzed, although insoluble and indigestible, are not in strict conformance with the definition of true keratins.
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