Oxytocin inactivation

Because of the metabolic lability of peptide bonds, metabolic inactivation is likely to play a particularly important role in the overall action of the peptide hormones. Structural alterations in the hormone molecule which increase metabolic stability would be expected to increase the duration of hormone action. Oxytocin is well known to be inactivated by an enzyme present in the serum of pregnant primates ( serum oxytocinase" see (1) but also more generally by homogenates of various tissues including the kidneys and liver. The major pathway of oxytocin inactivation by liver cell-sap has been shown to involve reductive fission of the disulphide bond followed by aminopeptidase degradation of the peptide chain (2). In an effort to obtain peptides resistant to aminopeptidase action we introduced D-hemicystine and N-methylhemicystine into the amino terminal position of... 

Position 4. Hydrolysis of the lateral amide group yields Glu -oxytocin with low activity. As this amide group is susceptible to hydrolysis, it is likely that the limited stability of the hormone under strongly acid or basic conditions is due to the conversion of oxytocin to Glu -oxytocin. Inversion of the amino acid (D-Glu -oxytocin) inactivates the molecule. 

Oxytocin (Pitocin, Syntocinon) is a cyclic 8-amino acid peptide that is synthesized in the paraventricular nucleus of the hypothalamus and transported within hypothalamic neurons (in association with neurophysin) to the posterior pituitary for storage. Its mechanism of action involves the direct stimulation of oxytocin receptors found on the myometrial cells. Oxytocin circulates unbound in the plasma, where it has a half-Ufe of approximately 15 minutes. It is primarily inactivated in the kidneys and liver. 

Sjoholm, I., Enzymatic inactivation of oxytocin. Acta Chem. Scand. 18 889-898 (1964). 

The importance of both the 20-member ring and the side chain for biological function has been investigated. These investigations are still in their infancy, and only tentative conclusions can be reached. We shall consider separately the structural changes that inactivate both the oxytocin and the vasopressive effects and those that are necessary to convert oxytocin into vasopressin. 

The presence of glutamine in position 4 and asparagine in position 5 is also important because the replacement by isoglutamine or isoasparagine inactivates the hormone. In fact, isoglutamine-4-oxytocin is an inhibitor of vasopressin. 

Two possible explanations of this situation come to mind. Either, there is some as yet unrecognised enzyme or enzyme system to which oxytocin, its deamino-analogues, and the carba-analogues are all equally susceptible (e. g,, an endopeptidase or amidase) and which plays a decisive role in the overall inactivation or, the invariance of the time course of the response is the result of some particular pharmacokinetic situation which makes this time course independent of the enzymic inactivation of the hormone. 

On the other hand, it has also been found (14) that 1-deamino-1-carba-oxytocin (Ib) is inactivated by liver homogenates, though very much more slowly than oxytocin. This finding demonstrates the existence of mechanisms other than disulphide reduction and aminopeptidase action capable of inactivating oxytocin-like peptides. The biochemical features and physiological significance of these mechanisms remain to be examined. 

Dhariwal 6e McCann, unpublished data, 1966). This observation indicates that intact peptide bonds are required for their activity. Carboxypeptidase failed to inactivate the FRF in one experiment. Thioglycollate splits the disulfide bridge in oxytocin and vasopressin, thus inactivating the molecules, but this treatment is without influence on LRF or hypothalamic CRF (16). Consequently, it appears likely that the chemical structures of LRF and CRF, at least with respect to the disulfide bridge, are dissimilar from that of the known neuroh3q)ophysial pol3q)eptides. 

Kidney and liver. The kidney and the liver play the major role in the clearance of oxytocin and vasopressin from the circulation. Liver extracts inactivate oxytocin by a two-stage process. The first step consists in the reduction of the disulfide bond with the simultaneous oxidation of reduced gluthathione, the reductive cleavage of oxytocin being coupled to the oxidation of NADPH as follows ... 

Oxytocinase (EC 3.4.1), an o-aminopeptide amino acid hydrolase which inactivates oxytocin, is present in blood during pregnancy. It contains sialic acid (Tuppy et al., 1963) which, according to Sjoholm... 

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