Decarboxylase inhibition and blood pressure reduction by alpha-methyl-3,4-dihydroxy-DL-phenylalanine

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Some Effects of Tranylcypromine and Trifluoperazine on Electrocortical Activity (EEG Activation)

In the deafferented cat devoid of the complicating effect of extraneous pharmacological agents (anaesthesia, muscle relaxants) the intra-innominate administration of tranylcypromine elicits immediate EEG activation. With high doses (7.5 mg/kg), EEG activation is immediate, intense, and persists for several hours. With lower doses (2.5 mg/kg) this effect is transient (20 seconds), giving way to a long period of deactivation (35–45 minutes), which in turn is followed by a secondary period of activation.

The immediate EEG activating response of tranylcypromine appears to be a direct effect, the secondary delayed phase of EEG activation appears to be an indirect action, such as might be expected from the retarded inactivation of central amines.

Trifluoperazine (2 mg/kg) elicits EEG deactivation with characteristics similar to those observed following the administration of “equi-deactivating” doses of chlorpromazine (5 mg/kg). When both trifluoperazine (2 mg/kg) and tranylcypromine (7.5 mg/kg) are introduced simultaneously, the activating effects of the latter dominate the earlier phases of the recording.

Preliminary experiments in some preparations in which gross unilateral lesions have been made in the mesencephalic tegmentum raises the possibility that this subcortical area may be a possible site for the arousal effect observed following the administration of tranylcypromine.

Enhanced functional connectivity and volume between cognitive and reward centers of naïve rodent brain produced by pro-dopaminergic agent KB220Z

Dopaminergic reward dysfunction in addictive behaviors is well supported in the literature. There is evidence that alterations in synchronous neural activity between brain regions subserving reward and various cognitive functions may significantly contribute to substance-related disorders. This study presents the first evidence showing that a pro-dopaminergic nutraceutical (KB220Z) significantly enhances, above placebo, functional connectivity between reward and cognitive brain areas in the rat. These include the nucleus accumbens, anterior cingulate gyrus, anterior thalamic nuclei, hippocampus, prelimbic and infralimbic loci. Significant functional connectivity, increased brain connectivity volume recruitment (potentially neuroplasticity), and dopaminergic functionality were found across the brain reward circuitry. Increases in functional connectivity were specific to these regions and were not broadly distributed across the brain. While these initial findings have been observed in drug naïve rodents, this robust, yet selective response implies clinical relevance for addicted individuals at risk for relapse, who show reductions in functional connectivity after protracted withdrawal. Future studies will evaluate KB220Z in animal models of addiction.

Renal Denervation

Sympathetic nervous system over-activity is closely linked with elevation of systemic blood pressure. Both animal and human studies suggest renal sympathetic nerves play an important role in this respect. Historically, modulation of sympathetic activity has been used to treat hypertension. More recently, catheter based renal sympathetic denervation was introduced for the management of treatment resistant hypertension. Sound physiological principles and surgical precedent underpin renal denervation as a therapy for treatment of resistant hypertension. Encouraging results of early studies led to a widespread adoption of the procedure for management of this condition. Subsequently a sham controlled randomised controlled study failed to confirm the benefit of renal denervation leading to a halt in its use in most countries in the world. However, critical analysis of the sham-controlled study indicates a number of flaws. A number of lessons have been learnt from this and other studies which need to be applied in future trials to ascertain the actual role of renal denervation in the management of treatment resistant hypertension before further implementation. This chapter deals with all these issues in detail.

[Hypotension and bradycardia before spinal anesthesia]

I report a case of hypotension and bradycardia before spinal anesthesia in a pregnant woman with mild to moderate hypertension treated with nifedipine and methyldopa, scheduled for an elective cesarean delivery. She had the history of neurally-mediated syncopes. Two main factors (increased vagal tone and adverse effects of antihypertensive drugs) could explain the hypotension and bradycardia before spinal anesthesia. Monitoring allowed recognizing the problem and corrected it. Thus, it was avoided a disaster in anesthesia, as hemodynamic changes after spinal anesthesia, they would have joined to previous hypotension and bradycardia, which would have caused even a cardiac arrest.

The role of the kidney and the sympathetic nervous system in hypertension

Nearly one-third of the world's population has hypertension. The human and societal impact of hypertension is enormous. Primary hypertension accounts for 95 % of cases of hypertension in adults. The pathogenesis of primary hypertension is complex. The kidney and the sympathetic nervous system play important roles in the development and maintenance of hypertension. This review discusses their respective roles, the interaction between the two, implications of sympathetic overactivity in kidney disease and therapeutic interventions that have been developed on the basis of this knowledge, especially modulation of the sympathetic nervous system.

Methyldopa for primary hypertension

BACKGROUND

Hypertension is associated with an increased risk of stroke, myocardial infarction and congestive heart failure. Methyldopa is a centrally acting antihypertensive agent, which was commonly used in the 1970's and 80's for blood pressure control. Its use at present has largely been replaced by antihypertensive drug classes with less side effects, but it is still used in developing countries due to its low cost. A review of its relative effectiveness compared to placebo on surrogate and clinical outcomes is justified.

OBJECTIVES

To quantify the effect of methyldopa compared to placebo in randomized controlled trials (RCTs) on all cause mortality, cardiovascular mortality, serious adverse events, myocardial infarctions, strokes, withdrawals due to adverse effects and blood pressure in patients with primary hypertension.

SEARCH STRATEGY

We searched the following databases: Cochrane Central Register of Controlled Trials (1960-June 2009), MEDLINE (2005-June 2009), and EMBASE (2007-June 2009). Bibliographic citations from retrieved studies were also reviewed. No language restrictions were applied.

SELECTION CRITERIA

We selected RCTs studying patients with primary hypertension. We excluded studies of patients with secondary hypertension or gestational hypertension.

DATA COLLECTION AND ANALYSIS

Two reviewers independently extracted data and assessed trial quality using the risk of bias tool. Data synthesis and analysis was performed using RevMan 5. Data for blood pressure were combined using the generic inverse variance method.

MAIN RESULTS

Twelve trials (N=595) met the inclusion criteria for this review. None of these studies evaluated the effects of methyldopa compared to placebo on mortality and morbidity outcomes. Data for withdrawals due to adverse effects were not reported in a way that permitted meaningful meta-analysis. Data from six of the twelve trials (N=231) were combined to evaluate the blood pressure lowering effects of methyldopa compared to placebo. This meta-analysis shows that methyldopa at doses ranging from 500-2250 mg daily lowers systolic and diastolic blood pressure by a mean of 13 (95%CI 6-20) / 8 (95% CI 4-13) mmHg. Overall, the risk of bias was considered moderate.

AUTHORS' CONCLUSIONS

Methyldopa lowers blood pressure to varying degrees compared to placebo for patients with primary hypertension. Its effect on clinical outcomes, however, remains uncertain.

Imidazoline antihypertensive drugs: a critical review on their mechanism of action

It was long thought that the prototypical centrally acting antihypertensive drug clonidine lowers sympathetic tone by activating alpha(2)-adrenoceptors in the brain stem. Supported by the development of two new centrally acting drugs, rilmenidine and moxonidine, the imidazoline hypothesis evolved recently. It assumes the existence of a new group of receptors, the imidazoline receptors, and attributes the sympathoinhibition to activation of I(1) imidazoline receptors in the medulla oblongata. This review analyzes the mechanism of action of clonidine-like drugs, with special attention given to the imidazoline hypothesis. Two conclusions are drawn. The first is that the arguments against the imidazoline hypothesis outweigh the observations that support it and that the sympathoinhibitory effects of clonidine-like drugs are best explained by activation of alpha(2)-adrenoceptors. The second conclusion is that this class of drugs lowers sympathetic tone not only by a primary action in cardiovascular regulatory centres in the medulla oblongata. Peripheral presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall sympathoinhibition.

Comparative bioavailability of alpha-methyldopa normal and film tablet formulations after single oral administration in healthy volunteers

In a single dose, randomized, cross-over study, with one week of wash-out period, the relative bioavailability of Dopegyt tablets containing 250 mg alpha-methyldopa (AMD) and Presinol film tablets with identical active ingredient content was examined in 24 healthy volunteers. Since technologically two completely different preparations (a film-tablet and a non-film-tablet) having significantly different in vitro dissolution were to be compared, both preparations were compared to a third one, AMD solution (Dopegyt solution) with 250 mg/50 ml concentration. Plasma concentrations of the drug were measured for 24 hours post-dose, applying HPLC with fluorometric detection. Pharmacokinetic parameters calculated from individual data (AUC0-infinity, AUC0-t, Cmax, Cmax/AUC0-infinity, t(max)) were evaluated statistically. Wilcoxon's nonparametric test and the four-way variance analysis could not detect any significant difference at the usual a=95% probability level in these pharmacokinetic parameters of the two tablet preparations. For AUC0-infinity at the 90% probability level, the confidence interval was 0.883-1.237 (with an estimated geometric mean of 1.045), for the test/reference ratio of Dopegyt and Presinol tablets, thus the two preparations proved to be bioequivalent. The relative bioavailability of Dopegyt (test preparation) and Presinol (reference preparation) calculated from the AUC0-infinity values was 116.7+/-56.7% that also confirmed bioequivalence. The results of all the applied statistical tests suggest that Dopegyt and Presinol can be considered as bioequivalent preparations.

Evolution of drugs that preserve renal function

Over the past 50 years, many advances have been made in slowing the progression of renal disease from various causes. These advances have been primarily linked to defining new lower levels for blood pressure goals as well as understanding the importance of proteinuria reduction. To achieve these goals, it is also appreciated that agents that lower blood pressure must also lower proteinuria. This is not true for all antihypertensive drug classes--notably, direct-acting vasodilators, alpha-blockers, and dihydropyridine calcium antagonists. Interestingly, antihypertensive agents that also reduce proteinuria have been associated with cardiovascular risk reduction. Moreover, an understanding of combinations of antihypertensive medications that provide additive reductions in proteinuria may be even more efficacious for slowing renal disease progression. It is hoped that these advances and the projected advances in pharmacogenetics will reduce the current increasing incidence of people going on dialysis.

Studies on the relationship of 5-hydroxytryptamine and the enterochromaffin cell to anaphylactic shock in mice

Protection against anaphylactic shock in mice by reserpine has been shown to be a delayed phenomenon, probably not dependent upon a direct effect of reserpine. The release and depletion of catechol amines by reserpine show little likelihood of being responsible for protection because these substances are in themselves protective against anaphylactic shock, while β-TM 10, a drug which interferes with their release is not. Since L-α-methyl dopa and reserpine both deplete serotonin, and since both protect against anaphylactic shock, it is proposed that serotonin depletion is responsible for the protection. Enterochromaffin substance is depleted in anaphylactic shock. It is also depleted by reserpine and serotonin, both of which protect against anaphylactic shock when given prior to challenge with antigen. The amount of enterochromaffin substance seems to correlate with susceptibility to anaphylactic shock. The behavior of animals undergoing anaphylactic shock and the effect of shock on body temperature is similar to the effects on behavior and temperature of treatment with reserpine, which is known to release serotonin. The effect of monoamine oxidase inhibition on animals undergoing anaphylactic shock is also similar to the effect of monoamine oxidase inhibition on animals given reserpine. These results are consistent with the views that the release of serotonin is causally related to anaphylactic shock in mice and that serotonin is accumulated in the chromogenic material of the enterochromaffin cell.

Drug management of hypertensive disorders of pregnancy

Drugs used in the acute and long-term management of hypertension in pregnancy and the preeclampsia-eclampsia syndrome have been reviewed and their therapeutic effects and maternal and fetal adverse effects have been considered. The review also focuses on recent developments in the areas of prevention and management of pre-eclampsia-eclampsia syndrome. Although a number of new drugs have emerged, as potentially useful in the management of hypertension in pregnancy and pre-eclampsia-eclampsia syndrome, some remain at the cornerstone of therapy; for example, methyldopa for long-term treatment of chronic hypertension, hydralazine or nifedipine for rapid reduction of severely elevated blood pressure, and magnesium sulphate for eclampsia. Some of these agents, especially the calcium antagonists, show promise in that their use is associated with fewer side effects. Safety for the fetus, however, has not been adequately evaluated yet. Neither aspirin nor calcium supplements appear to improve the outcome in pregnancy. Currently, the dilemma whether to treat hypertension in pregnancy and pre-eclampsia-eclampsia syndrome with old, established, cost-effective drugs or the promising newer drugs provides an interesting academic challenge.

REVERSAL OF THE EFFECT OF ALPHA-METHYLDOPA BY MONOAMINE OXIDASE INHIBITORS

L-α-Methyldopa, which normally causes sedation, induces a strong central excitation in mice pretreated with a monoamine oxidase inhibitor after a lag of a few hours. It is concluded that this excitation is caused by accumulation of free catecholamines liberated by amines which are slowly formed by decarboxylation of α-methyldopa. The hypotensive and sedative effects of α-methyldopa given alone are attributed to the slow release of catecholamines and subsequent breakdown by monoamine oxidase so that a partial depletion of catecholamines ensues.

SOME OBSERVATIONS ON THE PHARMACOLOGY OF ALPHA-METHYLDOPA

alpha-Methyldopa in high concentrations impaired the responses of rabbit isolated ileum and guinea-pig isolated vas deferens to stimulation of the sympathetic nerves and to noradrenaline, but these preparations taken from animals previously treated with alpha-methyldopa showed no sign of impairment. Contractions of the cat nictitating membrane were reduced but not abolished by alpha-methyldopa. In cats, dogs and rats, pressor responses to noradrenaline were usually slightly increased by alpha-methyldopa. Pressor responses to tyramine were not affected consistently. alpha-Methyldopa, alpha-methyldopamine and alpha-methylnoradrenaline behaved like dopa, dopamine and noradrenaline respectively in restoring the responses of tissues from reserpine-treated animals to stimulation of the sympathetic nerves to the rabbit ileum, the guinea-pig vas deferens and the cat nictitating membrane and in restoring responses to tyramine of the cat blood pressure and nictitating membrane, and the rat blood pressure. The potency of alpha-methylnoradrenaline relative to noradrenaline ranged from one-half to one-ninth on various preparations. The results are discussed in relation to the antihypertensive action of alpha-methyldopa.

Determination of alpha-methyldopa in human plasma by validated high-performance liquid chromatography with fluorescence detection

A sensitive reversed-phase gradient elution high-performance liquid chromatographic method with fluorescence detection has been developed for the determination of alpha-methyldopa (AMD) in human plasma. Separation of the investigated compound and the 3,4-dihydroxyphenylalanine (DOPA) internal standard was achieved on a Nucleosil 7 C18 column with a 5 mM heptanesulphonic acid sodium salt containing 0.05 M potassium dihydrogenphosphate (pH 3.2)-acetonitrile mobile phase. The composition of the mobile phase was changed according to a linear gradient time program. Detection was performed at 270 nm fluorimetric excitation and 320 nm emission. The compounds were isolated from plasma by Bond-Elut C18 solid-phase extraction. The limit of quantitation was found to be 10 ng/ml plasma. The assay was validated with respect to accuracy, precision and system suitability. All validated parameters were found to be within the 20% required limits. On the basis of the sensitivity, linearity and validation parameters the developed analytical method was found to be suitable for application in a bioequivalency study.

Lack of involvement of the kallikrein-kinin system in the depressor effect of alpha-methyldopa in normotensive rats

1. We evaluated the involvement of circulating kinins in the hypotensive effect of the antihypertensive drug alpha-methyldopa (alpha-MD) in normotensive rats. 2. The alpha-MD effects were more pronounced in urethane-anesthetized rats than in unanesthetized animals. 3. Treatment with alpha-MD did not affect the circulating levels of kininogen, the bradykinin precursor. 4. Pretreatment with captopril that potentiates the depressor effects of bradykinin did not potentiate the hypotension to alpha-MD. 5. The lack of changes in kininogen levels and the failure of captopril to potentiate the depressor effects of alpha-MD rules out an involvement of the circulating kallikrein-kinin system in the response to alpha-MD.

Immunogenicity studies of a synthetic antigen of alpha methyl dopa

Since the idiosyncratic liver toxicity of methyl dopa (L-alpha-methyl-3,4- dihydroxy-phenylalanine) may be due to immune mediated mechanisms, immunologic tools are needed to detect methyl dopa induced antibody and antigen. Hapten (methyl dopa)--carrier (albumin) conjugates were synthesized to generate antibodies reactive with this drug. Studies were also conducted to test the immunogenicity of this hapten-carrier conjugate and its cross reactivity with methyl catechol and levodopa. Methyl dopa (MD), levodopa (LD) or methyl catechol (MC) were conjugated to rabbit serum albumin (RSA) under high pH (base) conditions or by a tyrosinase (tyr) catalyzed reaction. Under the base conjugation conditions, MD-RSA, LD-RSA and MC-RSA conjugates were produced at higher hapten: carrier ratios than conjugates produced by the enzyme catalyzed reaction. Rabbits were subsequently immunized with either MD-RSA(base) or MD-RSA(tyr). Antibodies elicited by MD-RSA(base) had marked reactivity to the carrier protein, albumin, whereas antibodies elicited by MD-RSA(tyr) did not. In addition, reactivity of anti-MD antibody was equal to or greater with MC-RSA than reactivity with either MD-RSA or LD-RSA. This work suggests that the conjugation method using the tyr catalyzed reaction produces the optimal immunogen with minimal modification of the carrier protein. In addition, the catechol moiety of MD, MC and LD appears to be the immunogenic epitope on these haptens.

In-vivo quantitative imaging of catecholaminergic nerve terminals in brain and peripheral organs using positron emission tomography (PET)

Positron emission tomography (PET), has made possible quantitative imaging of the origins of positron-emitting isotopes, such as 11C and 18F, in intact animals and in humans. Lack of absolute specificity of enzymes, storage mechanisms and transporters allows 11C-or 18F-labelled "false transmitters" to be formed, stored and released from nerve terminals. Discussed are the assumptions, limitations, and advantages of 18F-6-fluoroDOPA, 18F-6-fluorodopamine, 18F-6-fluorometaraminol, and 11C-N-methyl-metaraminol (m-hydroxyephedrine) for PET imaging of dopaminergic nerve terminals in brain or sympathetic innervation in peripheral organs.

The evolution of antihypertensive therapy: an overview of four decades of experience

Hypertension is a major public health problem amendable to treatment. Numerous large scale clinical trials have demonstrated that effective, sustained control of elevated arterial pressure to a level below 140/90 mm Hg results in reduced cardiovascular morbidity and mortality. Over the past 4 decades antihypertensive drug therapy has evolved from a stepwise, but physiologically rational, selection of agents to specific programs tailored to individualized therapy for specific clinical situations. This evolution has taken place because of a greater understanding of the pathophysiology of hypertensive diseases, the development of new classes of antihypertensive agents that attack specific pressor mechanisms, and the ability to wed these concepts into a rational and specific therapeutic program. Thus, with the currently available spectrum of antihypertensive therapy, we are now able to select treatment for special patient populations utilizing a single agent and, therefore, we can protect the heart, brain and kidneys and maintain organ function without exacerbating associated diseases. These benefits are clear-cut and have resulted in many millions of patients becoming the beneficiaries of this transfer of careful, painstaking and purposeful investigative experiences into clinical practice.

Correlation between fall in blood pressure and in vivo amine release after alpha-methylDOPA

Blood pressure (BP), hypothalamic tissue concentrations and the in vivo overflow of endogenous and alpha-methylated catecholamines were measured in urethane anaesthetised rats after alpha-methylDOPA (mDOPA) administration (200 mg/kg i.p.). Four hours after mDOPA, BP fell to its lowest value, 60% of control, and slowly returned towards control levels by 24 h. This was closely correlated with the evoked overflow of alpha-methylnoradrenaline (mNA, r = 0.9) and noradrenaline (NA, r = 0.7) but not dopamine (DA) or alpha-methyldopamine (mDA). However, the tissue content of mNA rose much more gradually and was not maximal until after 12 h while mDA content followed the development of the hypotension. The results provide direct evidence for a false transmitter role for mNA in the brain, and suggest that the release of newly synthesised mNA is responsible for the hypotensive effect of mDOPA. Differences in the time course of overflow and storage of NA and mNA suggest the presence of separate transmitter storage and releasable pools.

Limitation of the alpha-methylnorepinephrine hypothesis in the hypotensive effect of alpha-methyldopa

To test the hypothesis that alpha-methylnorepinephrine (MNE) is the principal active metabolite involved in the hypotensive action of alpha-methyldopa (MD), we determined the relationship between MD's depressor response and tissue levels of MD metabolites in critical sites. After administration of 250 mg/kg MD intraperitoneally to Sprague-Dawley rats (300 +/- 50 g), we studied both heart (left ventricle) and brainstem MD, MNE and endogenous NE levels using HPLC with electrochemical detection. We also measured systolic blood pressure before and during MD (25-250 mg/kg i.p.) treatment using the tail-cuff method. Our results indicate that: (1) peak MD hypotensive response was dose-dependent. (2) Central NE concentration was maximally reduced by 2 hours whereas peripheral NE was maximally reduced by 18 hours. The maximal hypotensive effect was closer to the central peak distribution of MNE than MD. (3) The MD concentrations and NE concentrations in brainstem and heart showed counterclockwise hysteresis while MNE showed clockwise hysteresis. Furthermore, the area of MNE hysteresis in brainstem was larger than that of NE. We conclude that MD's depressor effect can not be completely explained by the assumption that MNE is the sole active metabolite; alternate metabolites or mechanisms would appear to be operative.

Side effects of sympatholytic antihypertensive drugs

Antihypertensive drugs with pharmacological action due to sympatholytic activity have been second only to diuretics in their use and efficacy in normalizing blood pressure. Their pharmacological actions have resulted in the notable absence of chemical toxicity, but because of symptomatic side effects, their use has been limited relative to some of the newer antihypertensive agents. Most prominent among undesirable side effects are the central nervous system findings of sedation, altered thought process, depression, and orthostatic or exercise hypotension. Sexual problems, especially in men, are also prominent. Special toxicity is discussed with reference to methyldopa, clonidine, monoamine oxidase inhibitors, and metyrosine.

Determination of alpha-alkyl-alpha-amino acids and alpha-amino alcohols by chiral-phase capillary gas chromatography and reverse-phase high-performance liquid chromatography

The enantiomeric resolution by fused-silica capillary gas-liquid chromatography (GLC) of non-protein DL-alpha-alkyl-alpha-amino acids of the structure H2NCR1R2COOH (R1 = alkyl, R2 = alkyl, alkaryl) was investigated by using chiral [L-valine-tert.-butylamide, linked to a statistical polymer of dimethylsiloxane and (2-carboxypropyl)methylsiloxane, Chirasil-L-Val, and XE-60-S-Val-S-alpha-phenylethylamide] and non-chiral (methylphenylcyanopropylvinylpolysiloxane, CP-Sil-19 stationary phases. To evaluate the resolution coefficients, N-acylamino acid n-propyl esters (acyl = acetyl, propionyl, trifluoroacetyl, pentafluoropropionyl, heptafluorobutyryl) and diastereomeric esters with S(-)-2-methyl-1-butanol, S(+)-2-butanol and S(+)-2-octanol were used. Although alpha-alkyl-alpha-amino acids in general gave lower resolution coefficients than the enantiomers of protein amino acids, most alpha-alkyl-alpha-amino acids could be resolved by using suitable derivatization procedures and, preferably, isothermal conditions. In addition, a number of DL-alpha-alkyl-alpha-amino acids could be separated by ligand-exchange chromatography (L-hydroxyproline/Cu2+) by both thin-layer chromatography (Chiralplate) and high-performance liquid chromatography (HPLC) (Nucleosil Chiral-1). Further, a standard mixture composed of fifteen alpha-amino acids and eleven alpha-amino alcohols could be completely separated by C18 HPLC after derivatization with o-phthaldialdehyde-2-mercaptoethanol (OPA-2-ME). The time and temperature dependences of the relative fluorescence of the adducts were investigated kinetically.