Histidase activity in hyperplastic and neoplastic rat epidermis and liver.

Histidase activity is first observed in rat epidermis just prior to birth and rises to a peak value at five days, which is the time when the epidermis undergoes maximum hyperplasia. In regenerating adult epidermis, histidase activity is similar to that of normal tissue. Benign epidermal tumors induced in an imals have normal histidase activity, but malignant ones have none. A similar pattern of histidase activity is seen in rat liver. The enzyme is absent during the fetal period and appears shortly after birth. Regenerating liver shows normal values of histidase activity, while enzyme activity cannot be detected in malignant tumors. The lack of enzyme activity results from an absence of histidase rather than the presence of an inhibitor. Since the histidine deaminase pathway has a different function in liver and epidermis, it is surprising that there is such a striking similarity between both tissues. Although fetal and malignant epidermis demonstrate the same lack of histidase, it has not been shown that the cause of this abnormality is the same for both tissues. In fetal epidermis there is an inhibition of histidase synthesis, but no data is available for malignant tumors.