Renin-angiotensin system: biochemistry and mechanisms of action.

There is both theoretical and therapeutic interest in establishing whether the signals conveyed by the enkephalins are turned off under the action of a specific peptidase which might, in this case, represent a target for a new class of psychoactive agents. Enkephalinase, a dipeptidyl carboxypeptidase cleaving the Gly3-Phe4 bond of enkephalins and distinct fropm angiotensin coverting enzyme (ACE), might be selectively involved in enkephalinergic transmission. It is a membrane-bound enzyme whose localization in the vicinity of opiate receptors in the central nervous system is suggested by parallel regional and subcellular distributions as well as by the effects of lesions. Such a role is further supported by the ontogenetic development of enkephalinase, its substrate specificity accounting for the increased biological activity of several enkephalin analogues and its adaptive increase following chronic treatment with morphine. To investigate the functional role of this enzyme further, we have designed a potent and specific enkephalinase inhibitor. We report here that this compound, thiorphan [(DL-3-mercapto-2-benzylpropanoyl)-glycine; patent no. 8008601] protects the enkephalins from the action of enkephalinase in vitro in nanomolar concentration and in vivo after either intracerebroventricular or systemic administration. In addition, thiorphan itself displays antinociceptive activity which is blocked by naloxone, an antagonist of opiate receptors.

The enkephalinase inhibitor thiorphan shows antinociceptive activity in mice

Many of the properties of angiotensin I converting enzyme or kininase II (ACE) have been discussed in extenso in the literature. The mode of action of ACE inhibitors has been studied in experimental animals and used clinically in millions of patients. The relatively few side effects have also been amply scrutinized. Nevertheless, some of the basic properties of this enzyme remain unexplained. For instance, although ACE was first considered to be a carboxypeptidase-type enzyme (peptidyl dipeptidase or dipeptidyl carboxypeptidase) (1,2) its actions go beyond cleaving dipep-tides from the free C-terminal end of peptide substrates. ACE inactivates substance P in spite of its blocked C-terminus, primarily by releasing the C-terminal tripeptide Gly-Leu-Met-NH2 (3). The blocked C-terminal tripep-tide, Arg-Pro-Gly-NH2, is also released from the luteinizing hormone releasing hormone (LHRH) (A). ACE, surprisingly, also cleaves the protected the N-terminal tripeptide <Glu -His -Trp from LHRH as well. A mechanism has been proposed (3) for the hydrolysis of C-terminal tripeptides from substrates with blocked C-terminal amino acids.

Angiotensin I converting enzyme.

The regional distribution and cellular localization of iron in the rat brain

Biological inactivation of enkephalins and the role of enkephalin-dipeptidyl-carboxypeptidase ("enkephalinase") as neuropeptidase.

— Angiotensin converting enzyme of rat brain was studied using Hip-His-Leu as substrate. The highest specific activity of the enzyme was associated with the microsomal fraction. The specific activity of the microsomal enzyme in several regions of the rat brain varied significantly. For example, the specific activities of the striatal and pituitary enzymes were about 10-fold greater than that of the cerebral cortical enzyme. The enzyme required chloride ion; moreover, activity was inhibited in the presence of disodium EDTA or O-phenanthroline, effects suggesting that the converting enzyme of brain is a metalloprotein. SQ-20881, a nonapeptide that inhibits converting enzyme in peripheral tissue, was a potent inhibitor of the enzyme of brain. In addition to Hip-His-Leu, the microsomal fraction was capable of liberating C terminal dipeptides from angiotensin I, Hip-Gly-Gly and Z-Gly- Gly-Val. The broad substrate specificity of the enzyme suggests that, in addition to the possible contribution of the enzyme to the brain renin-angiotensin system, other naturally occurring peptides might also be substrates for the enzyme.

H.‐Y. T. Yang

Papers

Distribution and properties of angiotensin converting enzyme of rat brain