Chemistry and Biology of Proteins - F. Haurowitz 1953

Proteins with Hormonal Activity
Other Protein-Based Hormones

Parathyroid hormone is extracted from parathyroid tissue using diluted Hydrochloric acid. Protein impurities that have passed into the solution are precipitated by adjusting the extract reaction to its isoelectric point (pH 5), after which the hormone is salted out under the same medium reaction [121]. The hormone can also be precipitated from the acid extract using acetone [122]. It is not denatured by diluted hot hydrochloric acid. One unit of the hormone is defined as the amount that raises the Blood calcium level in a 10 kg dog by 1 mg% over 16 hours. Pure hormone preparations contain approximately 300 units per 1 mg of nitrogen. Parathyroid hormone is apparently cleaved by Proteolytic Enzymes, as indicated by the lack of hormone activity when administered per os.

It is still not entirely clear whether the term "hormone" should be applied to renin, the so-called Kidney hormone. Renin is an enzyme-like substance that converts hypertensinogen into Hypertension [123]. Hypertensinogen is found in the plasma globulin fraction and is produced in the Liver [124]. Hypertension, in turn, is a polypeptide involved in the Regulation of Blood pressure [126]. It has been successfully isolated in crystalline form [125]. Hypertension (angiotonin) behaves toward proteolytic enzymes similarly to the synthetic polypeptide tyrosyl-lysyl-glutamyl-Tyrosine [127]. In all likelihood, renin is a proteolytic enzyme [128]. This is also indicated by the fact that Pepsin converts hypertensinogen into hypertension [129]. However, renin is not identical to renal cathepsin [130]. Hypertension is inactivated by a specific enzyme called hypertensinase [131].

Secretin is a proteinaceous substance secreted by the duodenal mucosa; it is generally regarded as an activator of pancreatic juice secretion. Secretin is cleaved by Trypsin; it is precipitated at half-saturation with ammonium sulfate and contains 13.8% nitrogen [132]. The Molecular Weight of secretin is less than 1,800, and its molecules do not contain free amino groups [133]. Since secretin is not produced in an endocrine gland, some researchers believe it should not be classified as a hormone. Similar doubts have been expressed regarding the so-called Hormones of the gastric and intestinal glands, as well as certain other Organs. The presence of these hormones is postulated based on their physiological action, yet to date none of them has been detected by chemical Methods [134].