Characteristics and specificity of phenelzine and benserazide as inhibitors of benzylamine oxidase and monoamine oxidase

Characteristics of Procarbazine as an Inhibitor In-vitro of Rat Semicarbazide-sensitive Amine Oxidase

Procarbazine (N-isopropyl-α-(2−methyl hydrazino)-p-toluamide hydrochloride) inhibited more powerfully the deamination of benzylamine by semicarbazide-sensitive amine oxidase (SSAO) of rat brown adipose tissue than the deamination of 5−hydroxytryptamine and benzylamine by rat liver monoamine oxidase-A or -B activities, respectively. Inhibition of SSAO, but not monoamine oxidase, was time-dependent. Use of metabolic inhibitors, and an enzyme dilution technique, suggested that any conversion of procarbazine to an active species must be as a result of the action of SSAO itself and not of any other enzyme. The non-competitive kinetics and the time-dependence of inhibition were indicative of a suicide interaction between procarbazine and SSAO. The slow reversal of inhibition by dialysis was evidence in favour of the involvement of tight binding, rather than covalent bonding. High concentrations of benzylamine afforded the enzyme significant protection from the action of procarbazine, indicating that the interaction is at or near the active site. If the properties of procarbazine, evident in in-vitro studies, are retained in-vivo, these data suggest that procarbazine might be suitable for the examination of SSAO activities, both in-vivo and ex-vivo.

The monoamine oxidase inhibitor phenelzine enhances the discriminative stimulus effect of nicotine in rats

In addition to delivering nicotine, tobacco smoke also inhibits monoamine oxidase (MAO). Although MAO inhibitors (MAOIs) can increase nicotine self-administration in rodents, the effects of MAOIs on the discriminative stimulus effect of nicotine are not known. This study examined the effects of three MAOIs (phenelzine, clorgyline and pargyline) with varying selectivity for MAOA and MAOB in the nicotine drug discrimination procedure in rats. Adult male Sprague-Dawley rats were trained to discriminate nicotine (0.3 mg/kg, subcutaneously) from saline in a standard, two-lever food-reinforced operant task. Once the discrimination was acquired, the ability of each MAOI to substitute for or alter the discriminative stimulus effect of nicotine was determined. In substitution tests, nicotine (0.03-0.3 mg/kg) produced full, dose-dependent substitution. Although the selective MAOA inhibitor clorgyline (3-56 mg/kg) and the selective MAOB inhibitor pargyline (3-56 mg/kg) did not elicit any nicotine-appropriate responding, partial substitution was obtained with the nonselective MAO inhibitor phenelzine (1-17 mg/kg). Phenelzine (10 mg/kg) also enhanced the discriminative stimulus effect of a low dose of nicotine (0.056 mg/kg) and prolonged the time course effect of the nicotine-training dose. These findings indicate that concomitant inhibition of MAOA and MAOB can enhance the discriminative stimulus effect of nicotine in rats.

Design, synthesis, and biological evaluation of semicarbazide-sensitive amine oxidase (SSAO) inhibitors with anti-inflammatory activity

In an attempt to examine the effect of inhibition of semicarbazide-sensitive amine oxidase (SSAO; EC 1.4.3.6, also known as VAP-1) as a novel anti-inflammatory target, the structure/mechanism based design and synthesis of a series of novel hydrazino-containing small molecules are described. The in vitro biological results show that compounds 4a,c are highly potent SSAO inhibitors with notable selectivity toward SSAO over monoamine oxidases A and B (MAO-A and MAO-B). SAR studies based on compound 4c were performed, and the results are discussed. The most potent and selective compound, 4a (IC(50) = 2 nM), is an orally active, competitive, and apparently irreversible inhibitor of SSAO that is effective at reducing disease incidence and severity in an in vivo animal disease model of multiple sclerosis.

Semicarbazide-sensitive amine oxidase: role in the vasculature and vasodilation after in situ inhibition

1. The characteristics of semicarbazide-sensitive amine oxidase (SSAO) are reviewed and the unknown physiological or pathological role of this enzyme emphasized. 2. The various mechanisms of action proposed for the vasodilator drug hydralazine are considered. In particular, the inhibitory action on various enzymes, related or not to cardiovascular function, are discussed. 3. Studies linking inhibition of SSAO to hydralazine hypotension are reviewed and a general hypothesis relating both actions is presented. The hypothesis postulates that (a). vascular SSAO is involved in the regulation of vascular tone, and (b). hydralazine vasodilation is the consequence of vascular SSAO inhibition. 4. Evidence supporting these postulates is presented and vascular SSAO inhibition is proposed as a novel mechanism of vasodilation.

Inhibition of bovine lung semicarbazide-sensitive amine oxidase (SSAO) by some hydrazine derivatives

Microsomal semicarbazide-sensitive amine oxidase (SSAO) from bovine lung was shown to be inhibited by a number of hydrazine derivatives, but the mechanisms of inhibition were found to differ. Hydralazine behaved as an irreversible and partially time-dependent inhibitor with an IC50 value of 1 microM under the conditions used. Phenylhydrazine was found to be a potent irreversible inhibitor of SSAO (IC50 30 nM). Semicarbazide behaved as a specific irreversible inhibitor (active-site-directed irreversible inhibitor) in first forming a non-covalent enzyme-semicarbazide complex (with a Ki value of 85 microM), which then reacted to give an irreversibly inhibited enzyme species in a reaction defined by the first-order rate constant k2 = 0.065 min-1. Phenelzine behaved as a reversible inhibitor, but dialysis at 37 degrees C was found to be necessary to obtain full recovery of enzyme activity. The dependence of inhibition on phenelzine concentration was complex and consistent with multiple binding sites for this inhibitor. This diversity in the action of a family of compounds with the same functional group must be taken into account in attempts to design more specific inhibitors of this enzyme.

Mammalian plasma and tissue-bound semicarbazide-sensitive amine oxidases: biochemical, pharmacological and toxicological aspects

Mammalian plasma and tissues contain various soluble and membrane-bound enzymes which metabolize the synthetic amine benzylamine particularly well. The sensitivity of these enzymes to inhibition by semicarbazide and related compounds suggests that they contain a cofactor with a reactive carbonyl group, which has been proposed to be either pyridoxal phosphate, pyrroloquinoline quinone or (more recently) 6-hydroxydopa. It is not yet clear if all of these semicarbazide-sensitive amine oxidases (SSAOs) are copper-dependent enzymes. A variety of compounds have now been identified as relatively selective inhibitors to distinguish the SSAOs from other amine oxidases, in order to investigate the properties of SSAOs and their potential role in biogenic and xenobiotic amine metabolism in vivo. While plasma SSAO is soluble, most tissue SSAOs appear to be membrane-bound, probably plasmalemmal enzymes, which may be capable of metabolizing extracellular amines. Vascular (and non-vascular) smooth muscle cells have particularly high SSAO activity, although recently the enzyme has been found in other cell types (e.g. adipocytes, chondrocytes, odontoblasts) implying a functional importance not restricted solely to smooth muscle. The substrate specificity of plasma and tissue SSAOs shows considerable species-related variations. For example, while some endogenously-occurring aromatic amines such as tyramine and tryptamine are metabolized well by SSAO in homogenates of rat blood vessels, and also in vitro inhibition of SSAO can potentiate vasoconstrictor actions of these amines in rat vascular preparations, these amines are poor substrates for human SSAO, thus complicating attempts to generalize possible physiological roles for these enzymes. Vascular SSAO can metabolize the xenobiotic aliphatic amine, allylamine, to the cytotoxic aldehyde acrolein and this has been linked to the ability of allylamine administration to produce cardiovascular lesions in experimental animals, sometimes mimicking features of atherosclerotic disease. Recent studies showing that the endogenously-occurring aliphatic amines methylamine and aminoacetone are metabolized in vitro to formaldehyde and methylglyoxal, respectively, by SSAO in some animal (including human) tissues, suggest the possibility that toxicological consequences upon cellular function could result if such conversions occur in vivo.

Further studies on the ex-vivo effects of procarbazine and monomethylhydrazine on rat semicarbazide-sensitive amine oxidase and monoamine oxidase activities

Following administration of the anticancer agent, procarbazine, or one of its metabolites, monomethylhydrazine, to rats, activities of monoamine oxidases A and B (MAO A and MAO B) and of semicarbazide-sensitive amine oxidase (SSAO) were measured ex-vivo. Both compounds were found to be potent inhibitors of SSAO in tissue homogenates, exhibiting ID50 values in most tissues of approximately 8 mg kg-1 (procarbazine) and 0.08 mg kg-1 (monomethylhydrazine). Concurrent dose-dependent inhibition of MAO activities did not occur. However, in liver, potentiation of MAO B activity, to 140% of that in controls, was apparent following monomethyl-hydrazine and this effect was independent of the drug dose. Both compounds produced a dose-dependent potentiation of MAO A in brown adipose tissue, the elevation being more pronounced following monomethylhydrazine, with activity rising to 350% of that in control homogenates. In a parallel in-vitro study, monomethylhydrazine was without effect on MAO A in brown adipose tissue homogenates. By perfusing the SSAO substrate, benzylamine, through the isolated mesenteric arterial bed of the rat, it was found that pretreatment of animals with procarbazine or monomethylhydrazine reduced metabolism of this amine by a similar degree as had been determined ex-vivo in blood vessel homogenates. The results presented suggest that these compounds would be suitable for use as selective inhibitors in pharmacological examinations of SSAO function in isolated tissues and organs.

Effects of long-term administration of phenelzine on reticular-elicited hippocampal rhythmical slow activity

All anxiolytics so far tested show a common reduction in the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). Acute administration of the tricyclic antidepressant imipramine shares this effect with anxiolytics. The present experiment tested whether the MAO inhibitor antidepressant phenelzine shares this common effect of anxiolytics and imipramine on hippocampal RSA. Rats, implanted with reticular stimulating electrodes and subicular recording electrodes, received four acute doses (0.2, 2.0, 6.0 and 18 mg/kg) or one chronic dose (2 mg/kg/day for 35 days) of phenelzine. Acute administration of phenelzine failed to systematically affect RSA frequency. Chronic injections of phenelzine eventually produced a reduction in RSA frequency combined with a gradual increase in baseline RSA frequency. The absence of immediate action and the production of a chronic reduction in RSA frequency are distinct from the documented effects of anxiolytics and imipramine, whereas the increase in baseline RSA frequency is similar to imipramine. The different influence of phenelzine on RSA frequency compared with anxiolytics (including imipramine) is consistent with the different clinical profiles of these drugs.

Substrate-specificity of mammalian tissue-bound semicarbazide-sensitive amine oxidase

Although the existence of a membrane-bound (probably plasmalemmal) semicarbazide-sensitive amine oxidase (SSAO) is well established in various mammalian tissues, and especially within vascular smooth muscle, its importance and the possible consequences of its metabolism of certain physiological and xenobiotic amines in vivo are under continuing investigation. In this respect, there are major species-related differences in substrate specificity determined in vitro, not only towards the synthetic amine benzylamine, but also towards some other aromatic amines (e.g. tyramine, tryptamine, 2-phenylethylamine, dopamine, histamine) which are possible endogenous substrates. Inhibition of SSAO can potentiate the pharmacological activity of some amines in isolated tissue (e.g. blood vessel) preparations from some species. Recent evidence has accumulated that SSAO may also be involved in metabolizing endogenous aliphatic amines such as methylamine and aminoacetone, focussing attention on the fact that the aldehyde products (formaldehyde and methylglyoxal, respectively) are potentially cytotoxic agents. Indeed, SSAO has been implicated in experimental models of cardiovascular toxicity involving conversion of the industrial aliphatic amine allylamine to acrolein. In summary, metabolism by SSAO may reduce the physiological/pharmacological effects of some amines, but the resulting metabolites (aldehydes, H2O2) may also have important actions.

Some aspects of the pathophysiology of semicarbazide-sensitive amine oxidase enzymes

The widespread distribution of enzymes classed as semicarbazide-sensitive amine oxidases (SSAO enzymes) throughout a very wide range of eukaryotic as well as prokaryotic organisms encourages the aspirations of those who wish to demonstrate physiological, pathological or pharmacological importance. Such enzymes are found in several tissues of mammals, both freely soluble, as in blood plasma, and membrane-bound, for example, in smooth muscle and adipose tissue. While they are capable of deaminating many amines with the production of an aldehyde and hydrogen peroxide, doubt still surrounds the identity of the most important endogenous substrates for these enzymes. At present, methylamine and aminoacetone appear to head the list of candidates. The possibility that SSAO enzymes can convert amine substrates to highly toxic metabolites is illustrated by the production of acrolein from the xenobiotic amine, allylamine and formaldehyde and methylglyoxal from methylamine and aminoacetone, respectively. Activities of SSAO enzymes may be influenced by physiological changes, such as pregnancy or pathologically by disease states, including diabetes, tumours and burns. Increased deamination of aminoacetone by tissue and plasma SSAO enzymes as a result of its increased production from L-threonine in conditions such as exhaustion, starvation and diabetes mellitus may be harmful. Such dangers could be mitigated either physiologically by a compensatory reduction in SSAO activity or pharmacologically by treatment with inhibitors of SSAO.

Effects of long-term administration of antidepressants on septal driving of hippocampal RSA

All classical anxiolytics raise the thresholds of septal-elicited hippocampal RSA overall, but do so mostly at 7.7 Hz (130 ms). The novel anxiolytic/antidepressant buspirone shows partial similarity with classical anxiolytics on septal driving thresholds. These effects of both the classical and novel anxiolytics are unchanged by long-term administration. The present experiment tested whether the tricyclic antidepressant imipramine and the monoamine oxidase inhibitor antidepressant phenelzine share these common effects of classical and novel anxiolytics with long-term administration. Rats, implanted with septal stimulating electrodes and subicular recording electrodes, received 15 mg/kg imipramine (twice per day) and 2 mg/kg phenelzine (once per day) for 28 days. Chronic administration of imipramine mimicked the documented effects of anxiolytics while chronic administration of phenelzine produced essentially opposite effects to the effects of anxiolytics on septal driving of RSA. Since both acute and chronic administration of imipramine but not phenelzine also produce similar effects to anxiolytics on the frequency of reticular-elicited hippocampal RSA, we suggest that (1) imipramine has a separate anxiolytic action, in addition to its antidepressant action; and (2) phenelzine may have no central anxiolytic action despite its capacity to relieve somatic symptoms in atypical depression.

A comparison of the acute effects of a tricyclic and a MAOI antidepressant on septal driving of hippocampal rhythmical slow activity

In free-moving male rats, the function relating frequency to the threshold current required to drive hippocampal rhythmical slow activity (RSA) with septal stimulation has a minimum at 130 ms. Both classical anxiolytics (e.g. benzodiazepines) and the novel anxiolytic buspirone show similar effects on septal driving of RSA. The tricyclic antidepressant imipramine may be as effective as anxiolytic drugs in treatment of generalized anxiety disorder. The antidepressant monoamine oxidase inhibitor phenelzine has also been reported to be effective in treating anxiety, but this may reflect an action on "atypical depression" rather than "anxiety". The present study therefore compared the effects of acute administration of imipramine and phenelzine on septal driving of RSA to determine whether either would mimic anxiolytics in this test. Rats were chronically implanted with septal stimulating electrodes and subicular recording electrodes. Three groups of rats received IP injection of either imipramine (5.9-13.3 mg/kg or 13.3-30 mg/kg) or phenelzine (0.2-5.4 mg/kg). The effects produced by imipramine were very similar to the effects produced by anxiolytic drugs. In contrast, the effects produced by phenelzine were essentially opposite to those of both anxiolytic drugs and imipramine. The present experiment suggests that imipramine may act as a true anxiolytic, in addition to its conventional antidepressant properties. In contrast, phenelzine may be effective in cases where the etiology is essentially that of depression even when the symptomatology appears to be that of anxiety.

Effect of pyridoxamine on semicarbazide-sensitive amine oxidase activity of rabbit lung and heart

Rabbit lung and heart show clorgyline-resistant benzylamine oxidase activity which is sensitive to semicarbazide (SSAO) and alpha-amino-guanidine. This SSAO activity is inhibited by pyridoxamine with an IC50 of 6.3 x -6 M for lung and of 1.1 x 10(-5) M for heart, the inhibition being non-competitive and only partially reversed by dialysis at 4 degrees C. Semicarbazide, alpha-aminoguanidine and pyridoxamine show a similar time-dependent type of inhibition of rabbit lung and heart SSAO.

Some properties of semicarbazide-sensitive amine oxidases

The semicarbazide-sensitive amine oxidases (SSAOs) comprise a substantial but diffuse group of enzymes separable from classical monoamine oxidase in several respects. Differences in cofactor requirement, molecular weight and subcellular distribution are crucial for such a separation. Differential sensitivity to enzyme inhibitors, characterized by resistance to inhibition by acetylenic MAO inhibitors coupled with sensitivity to semicarbazide and some related compounds are characteristic of these enzymes. SSAO enzymes have been found in the plasma of man, ox, pig and horse, for example as well as in the solid tissues of many species. Extensive studies have so far failed to produce any conclusive evidence to indicate what the precise functions of many of these enzymes may be. Indeed in most cases there is no clear idea as to the nature of the preferred physiological substrate, although many amines with pharmacological activity have been shown to be substrates. The actions of these amines may be potentiated following inhibition of SSAO, but as yet little is known whether or not these actions can be important in vivo. An attempt is made in this review to bring together some of the evidence to see if there are indications for future endeavours.

Vascular smooth muscle cells: a major source of the semicarbazide-sensitive amine oxidase of the rat aorta

Several methods have been used to study the distribution of the semicarbazide-sensitive amine oxidase (SSAO) within the wall of the rat aorta. After separation of the smooth muscle-containing layers of the tunica media from the connective tissue of the tunica adventitia, much higher specific enzyme activity (measured with 1 microM benzylamine) was found in homogenates of the media than of adventitia. Similar results were obtained for MAO-A (with 1 mM 5-HT as substrate). SSAO activity was also considerably higher in homogenates of cells (predominantly smooth muscle) isolated from medial tissue by enzymatic dissociation with collagenase and elastase compared with homogenates of cells (mostly of connective tissue origin) from the adventitia. Histochemical staining resulting from SSAO activity (with benzylamine as substrate) occurred predominantly and intensely over the tunica media in rat aortic sections, although some occasional staining of adventitial sites was also observed. Staining was prevented by the SSAO inhibitors hydroxylamine (1 microM) and semicarbazide (1 mM), but not by the MAO inhibitor, clorgyline (1 mM). These results indicate that SSAO is associated predominantly, although not exclusively, with the smooth muscle cells in the rat aorta. Our findings that beta-aminopropionitrile (BAPN) is a reversible, competitive inhibitor (Ki around 2 X 10(-4)M) of SSAO, in contrast to the irreversible inhibition of the connective tissue lysyl oxidase by BAPN reported by others, provides further evidence that these enzymes are not identical.

An allylamine derivative (MDL 72145) with potent irreversible inhibitory actions on rat aorta semicarbazide-sensitive amine oxidase

(E)-2-(3,4-dimethoxyphenyl)-3-fluoroallylamine (MDL 72145) was found to be an extremely potent inhibitor of the semicarbazide-sensitive amine oxidase (SSAO) in rat aorta homogenates. Considerable inhibition, which was not reversed by dialysis, could be produced under appropriate in-vitro conditions at drug concentrations around 10 nM. The pseudo first order kinetics for time-dependent inhibition by MDL 72145 (10-100 nM) were found to be consistent with a bimolecular reaction between enzyme and inhibitor with a rate constant for this step of 2 X 10(6) min-1 M-1. A similar rate of inhibition under an oxygen atmosphere to that obtained under nitrogen was produced upon incubation of enzyme with inhibitor, suggesting that oxidation of the inhibitor to an active metabolite was not required for its activity. Incubation of homogenates for very short periods (1 min) with inhibitor (0.05-0.5 microM) and benzylamine (1-10 microM) as substrate indicated non-competitive kinetics for the early interaction of enzyme with the drug. Benzylamine (50 microM), but not pyridoxal phosphate (100 microM), was able to protect SSAO from inhibition by 10 nM MDL 72145. However, pyridoxal phosphate alone appeared to produce some irreversible inhibition of the enzyme. Dialysis against buffer containing 50 microM or 1 mM benzylamine was unable to reactivate SSAO inhibited by 10 nM MDL 72145. It is concluded that MDL 72145 irreversibly inhibits SSAO by acting at, or near, the substrate binding site, but the exact nature of the complex formed remains to be identified.

Amine oxidase activities in brown adipose tissue of the rat: identification of semicarbazide-sensitive (clorgyline-resistant) activity at the fat cell membrane

Amine oxidase activity, previously described in homogenates of brown adipose tissue of the rat, has now been investigated in preparations of isolated fat cells. It was found that the specific activities of both monoamine oxidase A (MAO) and of the semicarbazide-sensitive clorgyline-resistant amine oxidase (SSAO) were higher in isolated fat cells than in the original whole tissue. Brown adipocytes therefore represent a major source of both these enzymes. In plasma membranes prepared from these isolated brown fat cells by borate extraction there was a similar enrichment of activity of SSAO and of the plasma membrane marker enzyme, phosphodiesterase I. However in preparations of cell membranes made by binding the cells to polycation-coated beads, enrichment of phosphodiesterase I activity was much greater than that of SSAO. It is suggested that the disposition of the enzyme within the cell membrane may account for the discrepancy in these results, i.e. the sidedness of the membrane may be important. Histochemical visualization of enzyme activity in whole tissue at the ultrastructural level was undertaken. Positive staining of mitochondria was achieved in the presence of the MAO substrate, tryptamine. Staining around the edges of the brown fat cells was observed with the SSAO substrates, tyramine and benzylamine. Staining was largely absent when substrate was omitted or after pretreatment with the irreversible SSAO inhibitor, hydralazine and the slowly reversible inhibitor, semicarbazide. It is not definitely proven that this staining represents sites of enzyme activity but the results are consistent with evidence from other studies indicating that SSAO in brown adipose tissue of the rat may be found predominantly at the fat cell surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in benzylamine oxidase activity in rat tissues

Brain, liver, heart, lung, kidney and duodenum benzylamine oxidase (BZAO) activities were measured from young and old rats. Protein content was found to decrease in liver (-17%), kidney (-20%) and duodenum (-17%) but remained unchanged in brain, heart and lung of old rats compared with that of young rats. A significant decrease (-41%) of BZAO activity was found in lung whereas a significant increase of enzyme activity was found in brain (+49%) and kidney (+25%) and no change was found in heart and duodenum of old rats. BZAO was not detected in either young or old rat liver. Kinetic analysis for lung BZAO activity of old rats showed that Vmax was decreased but Km was unchanged in comparison with that of young rats. Since, as we have shown previously, MAO-A and -B activity in lung of old rats was also found to be decreased, the decrease of lung BZAO activity with increasing age merits further investigation, lung playing an important role in removing amines from the circulation.

Inhibitory actions of hydralazine upon monoamine oxidizing enzymes in the rat

The inhibition by hydralazine of the clorgyline-resistant amine oxidase (CRAO) and monoamine oxidase (MAO) activities in various rat tissues has been studied. Hydralazine was a potent, time-dependent inhibitor of rat heart CRAO activity in vitro. The inhibition was not reversed by dialysis for 18 hr at 4 degrees, and only partially reversed by dialysis at 37 degrees. Dialysis at 4 degrees in the presence of pyridoxal phosphate (10(-4) M) also did not reverse the inhibition. Ex vivo inhibition of CRAO was found in heart and aorta homogenates in a dose-dependent manner after administration of hydralazine (1-40 mg/kg i.p.) to rats. In contrast, MAO-A activity was unaffected or, in some cases, significantly increased in these tissue homogenates from drug-treated animals. However, in vitro inhibition by hydralazine of both MAO-A and B activities of rat liver mitochondrial fractions was found, and these effects were fully reversible by dialysis for 18 hr at 4 degrees. Inhibition of MAO-A was competitive (Ki of 2.5 X 10(-6) M), while inhibition of MAO-B showed complex mixed non-competitive kinetics. These results indicate that hydralazine possesses different inhibitory properties towards the various amine oxidases in rat tissues, and these actions are discussed in relation to the clinical use of the drug as an anti-hypertensive agent.

Benzylamine oxidase in normal and atherosclerotic human aortae

Assays of serum benzylamine oxidase (BzAO) have led some workers to postulate a relationship between elevated BzAO activity and diseases characterized by proliferating connective tissue. The present study was designed to determine whether BzAO activity of a cellular tissue is also affected. BzAO was assayed in homogenates of normal and atherosclerotic human aortae. Characterization done in normal aortae showed that BzAO is not a classical monoamine, diamine, polyamine, or lysyl oxidase, nor is it a ceruloplasmin. The enzyme is heat stable at 60 degrees C and is associated primarily with the microsomal fraction on density centrifugation. Compared with phenylethylamines and indoleamines, benzylamine is the best substrate. BzAO is sensitive to inhibition by hydrazines and chymotrypsin but not trypsin, and is insensitive to Triton X-100 and sulfhydryl-group blockade. BzAO activity of atherosclerotic plaque (expressed per gram wet weight or per milligram protein) was decreased markedly compared to that in adjacent, nonplaque regions and in normal aortae. However, on a per milligram DNA basis, the BzAO activity of plaque did not differ from that of nonplaque tissue. We conclude that there is a decreased cell population density in plaque, a contention supported by kinetic analysis. Plaque BzAO showed a decreased Vmax with no change in the Km of benzylamine compared with nonplaque tissue. Thus, if a relationship exists between BzAO activity and proliferating connective tissue, it is not apparent at the level of the cellular enzyme in atherosclerotic aortae of man.