Possible role of octopamine and tyramine in the antihypertensive and antidepressant effects of tyrosine

The administration of a dose of 200 mg/kg of tyrosine (as either the free amino acid or the ethyl ester) increased the 24-hour excretion of p-hydroxyphenethyleneglycol (p-HPG) and p-hydroxyphenylethanol, metabolites of octopamine and tyramine, by 147 and 50%, respectively. One hour after this dose of tyrosine, brain levels of p-HPG and p-hydroxyphenylacetic acid (p-HPA), another metabolite of tyramine, were increased by 82 and 196%, respectively. Pretreatment with Ro4-4602, a peripheral decarboxylase inhibitor, reduced by 50% the tyrosine-induced increases in brain p-HPA levels, suggesting that tyramine was partially formed in the brain parenchyma. Tyrosine caused only slight, but non-significant increases in brain levels of catecholamine metabolites. These results suggest that tyrosine-induced increases in the production of tyramine and octopamine in brain may account for some of the effects of tyrosine, such as its antihypertensive and reported antidepressant properties.     

Induction of N-hydroxycinnamoyltyramine synthesis and tyramine N-hydroxycinnamoyltransferase (THT) activity by wounding in maize leaves

Both N-p-coumaroyl- and N-feruloyltyramine accumulated in response to wounding in leaf segments of maize. The amount of N-hydroxycinnamoyltyramines started to increase 3-6 h after wounding and peaked at 12 h. Thereafter, the amount of N-p-coumaroyltyramine decreased rapidly, while the N-feruloyltyramine content remained at a high level. The accumulation of N-hydroxycinnamoyltyramines was accompanied by an increase in the tyramine N-hydroxycinnamoyltransferase (THT) activity. This increase was initially detected 3 h after wounding and reached a maximum at 36 h, the level of activity being 40 and 11 times that in the leaves before wounding and in the control leaves, respectively. Partial purification of THT from wounded leaves by (NH4)2SO4 precipitation and subsequent two steps of anion-exchange chromatography resulted in a 12.5-fold increase in specific activity. Kinetic studies with this partially purified enzyme revealed that the best substrates were tyramine and feruloyl-CoA, although tryptamine and sinapoyl-CoA also efficiently served as substrates. The apparent native molecular weight of the enzyme was determined by gel filtration as 40 kDa.     

Biosynthesis and excretion of meta and para tyramine in the rat

$\text{Meta}$ and $\text{para}$ tyramine, after conversion to their bis dansyl derivatives, have been identified and quantitated by mass spectrometry in rat urine. The daily $\text{para}$ to $\text{meta}$ excretion ratio of 1.69 ± 0.10 is quite constant which suggests an endogenous origin for these amines. Following intraperitoneal injection of [¹⁴C] labelled dopa and dopamine more $\text{meta}$ than $\text{para}$ tyramine is excreted; after i.p. injection of $\text{para}$ tyrosine only small amounts of $\text{para}$ tyramine could be identified. This implies that some $\text{para}$ tyramine is synthesised by a route other than dehydroxylation or decarboxylation.     

TYRA-2 (F01E11.5): a Caenorhabditis elegans tyramine receptor expressed in the MC and NSM pharyngeal neurons

Tyramine appears to regulate key processes in nematodes, such as pharyngeal pumping, and more complex behaviors, such as foraging. Recently, a Caenorhabditis elegans tyramine receptor, SER-2, was identified that is involved in the TA-dependent regulation of these processes. In the present study, we have identified a second C. elegans gene, tyra-2 (F01E11.5) that encodes a tyramine receptor. This is the first identification of multiple tyramine receptor genes in any invertebrate. Membranes from COS-7 cells expressing TYRA-2 bind [(3)H]tyramine with high affinity with a K(d) of 20 +/- 5 nM. Other physiologically relevant biogenic amines, such as octopamine and dopamine, inhibit [(3)H]tyramine binding with much lower affinity (K(i)s of 1.55 +/- 0.5 and 1.78 +/- 0.6 microM, respectively), supporting the identification of TYRA-2 as a tyramine receptor. Indeed, tyramine also dramatically increases GTPgammaS binding to membranes from cells expressing TYRA-2 (EC(50) of 50 +/- 13 nM) and the TA-dependent GTPgammaS binding is PTX-sensitive suggesting that TYRA-2 may couple to Galpha(i/o). Based on fluorescence from tyra::gfp fusion constructs, TYRA-2 expression appears to be exclusively neuronal in the MC and NSM pharyngeal neurons, the AS family of amphid neurons and neurons in the nerve ring, body and tail. Taken together, these results suggest that TYRA-2 encodes a second Galpha(i/o)-coupled tyramine receptor and suggests that TA-dependent neuromodulation may be mediated by multiple receptors and more complex than previously appreciated.     

Regulation of derepressed synthesis of arylsulfatase by tyramine oxidase in Salmonella typhimurium.

The participation of tyramine oxidase in the regulation of arylsulfatase synthesis in Salmonella typhimurium was studied. Arylsulfatase synthesis was repressed by inorganic sulfate, cysteine, methionine, or taurine. This repression was relieved by tyramine, octopamine, or dopamine, which induced tyramine oxidase synthesis, although the level of arylsulfatase activity was very low. The induction of tyramine oxidase and derepression of arylsulfatase by tyramine were strongly inhibited by glucose and ammonium chloride, and the repression of both enzymes was relieved by use of xylose as a carbon source after consumption of glucose or by use of tyramine as the sole source of nitrogen, irrespective of the carbon source used. The initial rates of tyramine uptake by cells grown with glucose and xylose were similar. Results with tyramine oxidase-constitutive mutants showed that constitutive expression of the tyramine oxidase gene resulted in derepression of arylsulfatase synthesis in the absence of tyramine. Thus, catabolite and ammonium repressions of arylsulfatase synthesis and the induction of the enzyme by tyramine seem to reflect the levels of tyramine oxidase synthesis. These results in S. typhimurium support our previous finding that the specific regulation system of arylsulfatase synthesis by tyramine oxidase is conserved in enteric bacteria.     

Octopamine and tyramine influence the behavioral profile of locomotor activity in the honey bee (Apis mellifera)

The biogenic amines octopamine and tyramine are believed to play a number of important roles in the behavior of invertebrates including the regulation of motor function. To investigate the role of octopamine and tyramine in locomotor behavior in honey bees, subjects were injected with a range of concentrations of octopamine, tyramine, mianserin or yohimbine. Continuous observation of freely moving worker bees was used to examine the effects of these treatments on the amount of time honey bees spent engaged in different locomotor behaviors such as walking, grooming, fanning and flying. All treatments produced significant shifts in behavior. Decreases in time spent walking and increases in grooming or stopped behavior were observed for every drug. However, the pattern of the shift depended on drug, time after injection and concentration. Flying behavior was differentially affected with increases in flying seen in octopamine treated bees, whereas those receiving tyramine showed a decrease in flying. Taken together, these data provide evidence that octopamine and tyramine modulate motor function in the honey bee perhaps via interaction with central pattern generators or through effects on sensory perception.     

Properties of an enzyme from bananas (Musa sapientum) which hydroxylates tyramine to dopamine

An enzyme system isolated from the pulp of banana fruit (Musa sapientum) was partially purified and characterized. The enzyme was capable of catalysing the hydroxylation of the monophenol, tyramine, to the diphenol, dopamine (3,4-dihydroxyphenylethylamine). Unlike some tyrosinases, the reaction was not stimulated by catalytic amounts of diphenolic reaction product. Ascorbic acid, however, reduced the initial lag period in the oxidation of tyramine, stimulated the reaction rate and promoted the accumulation of dopamine during the first few minutes of the reaction. The hydroxylation of tyramine was apparently dependent upon molecular oxygen. On the basis of these observations it is tentatively suggested that the enzyme is a tyramine hydroxylase which may be responsible for the formation of dopamine in the banana.     

A comparison of the signalling properties of two tyramine receptors from Drosophila

In invertebrates, the phenolamines, tyramine and octopamine, mediate many functional roles usually associated with the catecholamines, noradrenaline and adrenaline, in vertebrates. The α‐ and β‐adrenergic classes of insect octopamine receptor are better activated by octopamine than tyramine. Similarly, the Tyramine 1 subgroup of receptors (or Octopamine/Tyramine receptors) are better activated by tyramine than octopamine. However, recently, a new Tyramine 2 subgroup of receptors was identified, which appears to be activated highly preferentially by tyramine. We examined immunocytochemically the ability of CG7431, the founding member of this subgroup from Drosophila melanogaster, to be internalized in transfected Chinese hamster ovary (CHO) cells by different agonists. It was only internalized after activation by tyramine. Conversely, the structurally related receptor, CG16766, was internalized by a number of biogenic amines, including octopamine, dopamine, noradrenaline, adrenaline, which also were able to elevate cyclic AMP levels. Studies with synthetic agonists and antagonists confirm that CG16766 has a different pharmacological profile to that of CG7431. Species orthologues of CG16766 were only found in Drosophila species, whereas orthologues of CG7431 could be identified in the genomes of a number of insect species. We propose that CG16766 represents a new group of tyramine receptors, which we have designated the Tyramine 3 receptors.     

Regulation of tyramine oxidase synthesis in Klebsiella aerogenes.

Tyramine oxidase in Klebsiella aerogenes is highly specific for tyramine, dopamine, octopamine, and norepinephrine, and its synthesis is induced specifically by these compounds. The enzyme is present in a membrane-bound form. The Km value for tyramine is 9 X 10(-4) M. Tyramine oxidase synthesis was subjected to catabolite repression by glucose in the presence of ammonium salts. Addition of cyclic adenosine 3',5'-monophosphate (cAMP) overcame the catabolite repression. A mutant strain, K711, which can produce a high level of beta-galactosidase in the presence of glucose and ammonium chloride, can also synthesize tyramine oxidase and histidase in the presence of inducer in glucose ammonium medium. Catabolite repression of tyramine oxidase synthesis was relieved when the cells were grown under conditions of nitrogen limitation, whereas beta-galactosidase was strongly repressed under these conditions. A cAMP-requiring mutant, MK54, synthesized tyramine oxidase rapidly when tyramine was used as the sole source of nitrogen in the absence of cAMP. However, a glutamine synthetase-constitutive mutant, MK94, failed to synthesize tyramine oxidase in the presence of glucose and ammonium chloride, although it synthesized histidase rapidly under these conditions. These results suggest that catabolite repression of tyramine oxidase synthesis in K. aerogenes is regulated by the intracellular level of cAMP and an unknown cytoplasmic factor that acts independently of cAMP and is formed under conditions of nitrogen limitation.     

Peroxidase-mediated integration of tyramine into xylem cell walls of tobacco leaves

When [2-¹⁴C]tyramine was fed in vivo by petiolar uptake to Nicotiana tabacum Xanthi n.c. leaves partially inoculated with tobacco mosaic virus, radioactivity accumulated in inoculated areas bearing necrotic lesions, mainly in the veins and around the lesions. Light-microscopic autoradiography showed that integration of radioactivity was especially evident in xylem cell walls. This was confirmed in sections of petiole by electron-microscopic autoradiography. Study of the mechanism of insolubilisation of tyramine showed that the amine was integrated in regions in which peroxidase activity could be located cytochemically using 3,3-diaminobenzidine and H₂O₂ as substrates. When sections of petiole were incubated with labelled tyramine and H₂O₂ after fixation in glutaraldehyde, a distribution of radioactivity similar to that obtained after feeding tyramine by petiolar uptake was observed. It is concluded that simple phenols such as tyramine can be integrated in vivo into cell walls because they are oxidised by peroxidases. This result illustrates the difficulty of studying the metabolism of exogenous phenols in plants, especially in lignifying tissues which contain active wall-bound peroxidases.Abbreviations DAB 3,3-diamino-benzidine - TMV tobacco mosaic virus     

Implication of tyramine in the biosynthesis of morphinan alkaloids in Papaver

Doubly-labeled [³H, ¹⁴C]tyrosines, [1-¹³C-]tyramine or [2-¹⁴C]tyramine, administered to the stems of intact Papaver somniferum L. plants, were found to be incorporated into the morphinan alkaloids of the plant with comparable efficiency. ³H/¹⁴C ratios of alkaloids from plants fed the tyrosines were consistent with an almost equal conversion of this amino acid into the tetrahydroisoquinoline (TIQ) and benzyl-derived segments. Nuclear magnetic resonance (NMR) analyses of morphine isolated after administration of [1-¹³C]tyramine demonstrated selective labeling of C-16 of the alkaloid, indicating the conversion of this amine primarily into the TIQ-derived moiety. Morphine and thebaine labeled by [2-¹⁴C]tyramine were degraded to phenanthridines and N,N-dimethyl ethylamines. Of the total radioactivity in the alkaloids 97% was found to be associated with the ethylamines, a distribution consistent with the NMR data. This preferential utilization of tyramine in the biosynthesis of morphinan alkaloids can be explained by the compartmentalization of intermediates and enzymes of the pathway.Abbreviations L-dopa L-3,4-dihydroxyphenylalanine - HPLC high-pressure liquid chromatography - NMR nuelear magnetic resonance - TIQ tetrahydroisoquinoline     

A new family of insect tyramine receptors

The Drosophila Genome Project database contains a gene, CG7431, annotated to be an "unclassifiable biogenic amine receptor." We have cloned this gene and expressed it in Chinese hamster ovary cells. After testing various ligands for G protein-coupled receptors, we found that the receptor was specifically activated by tyramine (EC(50), 5x10(-7)M) and that it showed no cross-reactivity with beta-phenylethylamine, octopamine, dopa, dopamine, adrenaline, noradrenaline, tryptamine, serotonin, histamine, and a library of 20 Drosophila neuropeptides (all tested in concentrations up to 10(-5) or 10(-4)M). The receptor was also expressed in Xenopus oocytes, where it was, again, specifically activated by tyramine with an EC(50) of 3x10(-7)M. Northern blots showed that the receptor is already expressed in 8-hour-old embryos and that it continues to be expressed in all subsequent developmental stages. Adult flies express the receptor both in the head and body (thorax/abdomen) parts. In addition to the Drosophila tyramine receptor gene, CG7431, we found another closely related Drosophila gene, CG16766, that probably also codes for a tyramine receptor. Furthermore, we annotated similar tyramine-like receptor genes in the genomic databases from the malaria mosquito Anopheles gambiae and the honeybee Apis mellifera. These four tyramine or tyramine-like receptors constitute a new receptor family that is phylogenetically distinct from the previously identified insect octopamine/tyramine receptors. The Drosophila tyramine receptor is, to our knowledge, the first cloned insect G protein-coupled receptor that appears to be fully specific for tyramine.     

The effect of various monoamine oxidase (MAO) inhibitors on the response of blood pressure of rats and cats to tyramine

The "cheese effect" is the clinically most important side effect of structurally different MAO inhibitors. It occurs mainly as a result of the interaction of MAO inhibitor with tyramine in foodstuffs. Anaesthetised rats and cats were used in order to investigate and compare the influence of the effect of tyramine by selective MAO type-B inhibitors with that produced by non-selective and A-selective MAO inhibitors on the one hand, and on the other hand, different MAO-B inhibitors with (-)deprenyl. (-)Deprenyl was the only one which inhibited the effect of tyramine in the experimental animals used, while other MAO inhibitors potentiated the tyramine effect. Therefore this study indicates that not only non-selective and A-selective inhibitors potentiate the effect of tyramine but selective inhibitors of B-type MAO as well. The inhibition of tyramine uptake by (-)deprenyl is a remarkable exception from the rule.     

Potential effects of tyramine on the transition to reproductive workers in honeybees (Apis mellifera L.)

Abstract.  To explore the role of brain tyramine in reproductive worker honeybees, its effects after injection and oral treatment on brain dopamine levels and ovarian development in queenless worker honeybees are determined. Both tyramine injection and oral treatment in 10-day-old queenless bees leads to tyramine transportation into the brain and significantly elevates brain dopamine levels as a function of the tyramine concentration. Ovarian diameters are significantly larger in 10-day-old queenless bees treated with tyramine compared with queenless bees of the same age without tyramine treatment. Results on yolk formation in the ovary support the finding of increased ovarian diameter, suggesting that oral tyramine treatment accelerates ovarian development through dopamine effects and/or direct effects of tyramine on the ovary in queenless bees. Thus, tyramine has potential effects on the enhancement of brain dopamine levels and the acceleration of ovarian development for the transition of normal workers to reproductive worker honeybees.     

A tyramine receptor gene mutation causes a defective olfactory behavior in Drosophila melanogaster

We characterized molecular profiles of a new olfactory mutant line, honoka (hono), which was found among 500 viable P-element insertion lines screened first by 5-bromo-4-chloro-3-indrolyl-beta-D-galactopyranoside (X-gal) staining on the third segment of the antenna, and then by behavioral assays to several pure chemicals.The behavioral responses of hono mutants to repellents such as ethyl acetate (EA), benzaldehyde (BZ) and 4-methylcycrohexanol (MCH), were reduced compared with those of a control strain. The location of the P-element insertion was determined to be about 100bp) upstream of the first exon of the tyramine receptor gene. The level of 3.6kb tyramine receptor mRNA expression was reduced in hono compared with that of wild-type flies. The tyramine receptor cDNA hybridized to the chromosomal division 79C-D, the same locus as the P-element insertion point in hono, and not to 99A-B, previously reported by Arakawa et al. (1990. Neuron 2, 343-354).Electrophysiological responses to octopamine and tyramine were examined by measuring the excitatory junctional potential (EJP) amplitude from larval body-wall muscles of the hono mutant. The hono was impaired with responding to tyramine, while displaying normal response to octopamine. These results indicate that tyramine has a functional role in the Drosophila olfactory system as a neurotransmitter or a neuromodulator, and hono is the first tyramine receptor mutant. This study provides the first step toward understanding of the molecular genetics of tyramine-mediated neural functions in Drosophila.     

Mechanism of inhibition by chlorgyline and deprenyl of tyramine deamination by mitochondrial monoamine oxidase of rat liver

The inhibition by chlorgyline and deprenyl of deamination of tyramine, i. e. substrate of two forms of monoamine oxidase (MAO) A and B, by fragments of rat liver mitochondrial membrane and the effects of competitive reversible inhibitors of the MAO activity, e. g. 4-ethylpyridine, benzyl alcohol, O-benzyl-hydroxylamine and 2-oxyquinoline, on this process were studied. It was shown that all the inhibitors used sharply increase the inhibiting effect of chlorgyline on tyramine deamination, the degree of the stimulating effect being the same irrespective of whether the inhibitors are added to the samples before or after a 30-min preincubation of chlorgyline with the enzyme at 23 degrees, i. e. after the onset of irreversible inhibition. The stimulating effect is due to the independent action of two inhibitors on the two different sites of the MAO active center: chlorgyline--on the isoalloxazine ring of FAD, that of 4-ethylpyridine, benzyl alcohol, O-benzylhydroxylamine, 2-oxyquinoline, respectively, on the hydrophobic region involved in tyramine binding. In similar experiments with deprenyl all the competitive inhibitors used, when added to the samples after a 30-min incubation of the inhibitor with the enzyme at 23 degrees, remove the inhibiting effect of deprenyl on tyramine deamination. The decrease of the inhibiting effect of deprenyl is indicative of an existence of competitive interactions between deprenyl and the above-mentioned compounds and of the reversible inhibition by deprenyl of tyramine deamination under the given experimental conditions. The data obtained revealed the differences in the type and mechanism of action of chlorgyline and deprenyl on tyramine deamination and showed that these inhibitors act on different sites of the MAO active center, responsible for tyramine oxidation. Chlorgyline blocks primarily the "flavin moiety" of the MAO molecule, essential for the catalytic act, while the effect of deprenyl is directed to the hydrophobic part of the enzyme active center essential for the enzyme binding to tyramine. In this case the irreversible inhibiting effect is achieved at a slower rate and the reversibility of tyramine oxidation by deprenyl is maintained for a longer period of time than the chlorgyline inhibition of deamination of this amine.     

Modulation of tyramine signaling by osmolality in an insect secretory epithelium

The control of water balance in multicellular organisms depends on absorptive and secretory processes across epithelia. This study concerns the effects of osmolality on the function of the Malpighian tubules (MTs), a major component of the insect excretory system. Previous work has shown that the biogenic amine tyramine increases transepithelial chloride conductance and urine secretion in Drosophila MTs. This study demonstrates that the response of MTs to tyramine, as measured by the depolarization of the transepithelial potential (TEP), is modulated by the osmolality of the surrounding medium. An increase in osmolality caused decreased tyramine sensitivity, whereas a decrease in osmolality resulted in increased tyramine sensitivity; changes in osmolality of ±20% resulted in a nearly 10-fold modulation of the response to 10 nM tyramine. The activity of another diuretic agent, leucokinin, was similarly sensitive to osmolality, suggesting that the modulation occurs downstream of the tyramine receptor. In response to continuous tyramine signaling, as likely occurs in vivo, the TEP oscillates, and an increase in osmolality lengthened the period of these oscillations. Increased osmolality also caused a decrease in the rate of urine production; this decrease was attenuated by the tyraminergic antagonist yohimbine. A model is proposed in which this modulation of tyramine signaling enhances the conservation of body water during dehydration stress. The modulation of ligand signaling is a novel effect of osmolality and may be a widespread mechanism through which epithelia respond to changes in their environment. Drosophila; Malpighian tubule; cell volume regulation; G protein-coupled receptor; biogenic amines     

Effect of prolonged treatment with tyramine on glucose tolerance in streptozotocin-induced diabetic rats

The biogenic amine tyramine has been reported to stimulate in vitro glucose transport in adipocytes, cardiomyocytes and skeletal muscle, and to improve in vivo glucose utilization in rats. These effects were dependent on amine oxidation, since they were blocked by inhibitors of monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO). We thus tested in this work whether a prolonged treatment with tyramine could improve glucose tolerance in streptozotocin-induced diabetic rats. First, tyramine content of standard rodent chow was determined by HPLC and daily tyramine intake of control rats was estimated to be around 26 micromol/kg body weight. Then, tyramine was administred during 3 weeks in streptozotocin-induced diabetic rats at 29 micromol/kg by daily i.p. injection alone or together with vanadate 0.02 micromol/kg. In another group of diabetic rats, tyramine was subcutaneously delivered at 116 micromol/kg/day by osmotic minipumps. All tyramine treatments resulted in a decrease of the hyperglycemic responses to an i.p. glucose load. Adipocytes isolated from either untreated or treated diabetic rats were sensitive to the stimulation of glucose uptake by tyramine. However, diabetic animals receiving tyramine for three weeks did not recover from their hyperglycemia, hypoinsulinemia and glucosuria. These results show that the improvement of glucose tolerance induced by prolonged tyramine administration occurs in an insulin-depleted model and probably results from peripheral insulin-like actions of the oxidation of MAO/SSAO substrates, such as the stimulation of glucose uptake into adipocytes.     

Influence of the biogenic amine tyramine on ethanol-induced behaviors in Drosophila

The biogenic amine tyramine has been implicated in drug-induced behavior. The Drosophila inactive mutant is characterized by reduced tyramine and octopamine levels and is defective in cocaine sensitization. To test whether there is an overlap in the use of the amine neurotransmitter system in ethanol- and cocaine-induced behaviors, mutant analyses were extended to the phenotypic characterization of inactive and other mutants effecting the tyramine and octopamine neurotransmitter system. The inactive mutant displays increased ethanol sensitivity and is impaired in the initial startle response upon ethanol application. Furthermore, this mutant fails to regulate its alcohol-induced hyperactivity properly. In contrast to the defects seen after cocaine application, inactive mutants develop normal ethanol tolerance and sensitize to the locomotor activating effect of ethanol. The tyramine-beta-hydroxylase mutant (TbetaH) with increased tyramine and depleted octopamine levels displays normal ethanol sensitivity, a startle repression, and hyperactivates more in response to ethanol. In addition, TbetaH mutants fail to develop a tolerance to the hyperactivating effect of ethanol. Ethanol-induced sensitization does not seem to be impaired in either mutant, suggesting that tyramine is not required for this process. The comparative analysis of the phenotypes associated with inactive and TbetaH mutants suggests that the fine tuning of ethanol-induced hyperactivity can be correlated with different tyramine levels. Defects in other aspects of ethanol-induced behaviors might be due to different molecules or mechanisms.     

The conjugation of tyramine with sulphate by liver and intestine of different animals.

Tyramine was conjugated with sulphate by extracts of monkey intestine and livers of monkey, rat, mouse, guinea pig and man. The activity measured in monkey intestine was almost three times that of monkey liver. Labelled tyramine sulphate synthesized from [14C] tyramine, [3H] tyramine or Na235SO4, on acid hydrolysis, released its radioactive precursor. Liver extracts of monkey, rat, mouse and guinea pig synthesized respectively 145,66,21 and 6 pmol of [14C] tyramine sulphate/min per mg of protein. Except with the monkey, intestine exhibited very low activity. trans-2-Phenylcyclopropylamine, a monoamine oxidase inhibitor, was added as a routine to the enzyme preparation, as its omission resulted in the production of p-hydroxyphenylacetic acid in appreciable amounts. This oxidative deamination of tyramine, however, did not decrease the sulpho-conjugation of tyramine. The low Km (9.1 muM) of sulphotransferase for tyramine is probably responsible.