Benserazide

Introduction

Vitamin B6 consists of three related pyrimidine vitamer derivates: pyridoxine, pyridoxal, and pyridoxamine, and their phosphate esters. The metabolically active coenzyme form of vitamin B6 is pyridoxal 5'-phosphate (P5P).[1] B6 vitamers are first oxidized to pyridoxal, and rapidly phosphorylated to P5P in the liver.[2] P5P is the main circulating form exported from the liver and is considered the most relevant direct measure of vitamin B6 status.[3]

Pyridoxine was first isolated in 1934 and named by Albert Szent-Gyorgy.[1] The standard B6 vitamin supplement is pyridoxine hydrochloride (HC1), which is the least expensive to produce commercially;[4] however, P5P is the only form which can be used by the enzymes involved in biochemical processes, which are associated with nitrogen and protein metabolism and heme synthesis.[5] Plasma P5P levels were found to be significantly lower than normal in 22 out of 31 patients with impaired liver function, which reflects the liver's importance in B6 conversion. In patients receiving pyridoxine HC1, only 33 percent responded with an increase in plasma P5P, while all of the patients receiving P5P responded with an increase.[6]

Biochemistry/Pharmokinetics

Pyridoxine is water soluble and stable in heat and acid mediums, while it is unstable in alkaline solutions and light.[7] Pyridoxine and its vitamers are absorbed in the upper small intestine by simple diffusion and transported to the liver for biotransformation into the active coenzyme P5P, which is then exported from the liver bound to albumin. Uptake into tissue is by extracellular de-phosphorylation, followed by metabolic trapping intracellularly as P5P.[8]

P5P-dependent enzymes are involved in the following reactions:[8]

* decarboxylation of amino acids to yield amines, many of which are important neurotransmitters and hormones.

* transamination of amino acids to keto-acids, which are then oxidized and used as metabolic fuel.

* phosphorolytic cleavage of glycogen (from liver and muscle) to glucose-one-phosphate.

* formation of alpha aminolevulinic acid, a precursor to heme.

* decarboxylation of phosphatidylserine to phosphatidylethanolamine in phospholipid synthesis.

* as a co-factor in a variety of reactions involving side-chain cleavage, including cystathionine synthase and cystathionase.

Clinical Applications

Anemia

P5P is necessary for the activation of glycine in the initial stages of heme production. Several cases of B6-responsive anemias have shown no response to pyridoxine and prompt response to P5P. This suggests an enzymatic deficiency or inhibition of pyridoxal kinase.[9] In another study, excessive alcohol produced bone marrow sideroblastic changes that were responsive to P5P, while no response was achieved from pyridoxine or folic acid.[10] From a therapeutic point of view, P5P should be tried in all cases of symptomatic primary sideroblastic anemia which have shown no response to pyridoxine.[9]

In 16 patients with sickle cell anemia, plasma P5P concentrations were significantly lower than in 16 controls. Oral supplementation of five of the patients with 50 mg pyridoxine twice daily for two months resulted in increased plasma P5P levels.[11] Since pyridoxine and P5P have been shown to have anti-sickling properties in vitro,[12,13] these studies suggest P5P supplementation may also be of therapeutic benefit in vivo in sickle cell anemia.[11]

Carpal Tunnel Syndrome

Researchers have found a direct correlation between carpal tunnel syndrome (CTS) and a deficiency in P5P, and that treatment with 100-200 mg pyridoxine daily for at least 12 weeks was highly beneficial in reducing both the symptomology and a P5P deficiency associated with CTS.[14-16] A few studies have shown no clinical benefit from pyridoxine HC1 supplementation,[17-19] with most of the reports of beneficial effects coming from Ellis et al.[15,20-23] A 1996 literature review of clinical trials using pyridoxine to treat CTS concluded that the evidence for the use of pyridoxine as the sole treatment for CTS is weak, but that it may be valuable as an adjunctive treatment through its effect on altered perception of pain and increased pain threshold.[24]

Premenstrual Syndrome (PMS)

B6 nutritional status has a significant and selective modulatory impact on central production of both serotonin and GABA -- neurotransmitters that control depression, pain perception, and anxiety.[25] P5P is a cofactor in the synthesis of these neurotransmitters.[26] Authors of a 1999 review of published and unpublished randomized placebo-controlled trials of the effectiveness of vitamin B-6 in the management of PMS concluded that the pooled data of nine trials representing 940 patients suggests doses ofpyridoxine up to 100 mg/day are likely to be of benefit in treating premenstrual symptoms, including premenstrual depression.[27]

Hyperhomocysteinemia

High levels of plasma homocysteine are considered an independent risk factor for atherosclerotic disease and venous thrombosis. Homocysteine, an intermediate in methionine metabolism, can be re-methylated to methionine, or be channeled down the trans-sulfuration pathway to cysteine, which requires two P5P-dependent enzymes: cystathionine synthase and cystathionase.[8]

A study of 1,160 elderly subjects (67-96 years) correlated high homocysteine levels with low folate and B6.[28] Results of the 1998 Nurses' Health Study showed cardiovascular disease risk was lowest among those women with the highest intakes of folate and B6.[29] Vitamin therapy -- a combination of folic acid (500 mcg) and pyridoxine (100 mg) -- in 49 hyperhomocysteinemic persons, significantly reduced fasting plasma homocysteine concentrations (median 13.9 to 9.3 mM/L, reduction 32%) and post-methionine load concentrations (median 55.2 to 36.5 mM/L, reduction 30%).[30]

Studies that have tried to distinguish between the potential beneficial effects of folate vs B6 have shown folate to be more efficacious in lowering serum homocysteine. One study showed that, while folic acid 650 mcg/day lowered fasting homocysteine in moderately hyperhomocysteinemic subjects, 10 mg B6/day had no effect.[31] A similar study showed folic acid 400 mcg/day reduced plasma homocysteine in people with non-elevated homocysteine levels while pyridoxine 2 mg/day had no effect.[32] It should be noted, however, that 10 mg/day and 2 mg/day B6 are quite low doses.

Other Clinical Indications

A double-blind trial using pyridoxine (25 mg every eight hours for three days) in the treatment of morning sickness resulted in a significant reduction in vomiting, and an improvement in nausea in those who initially reported severe nausea.[33]

Researchers in Japan published animal studies suggesting that B6 deficiencies impair conversion of alpha-linolenic acid to EPA and DHA, with the most pronounced reduction in production of DHA.[34]

P5P has also been shown to protect rat kidneys from the nephrotoxicity of aminoglycoside antibiotics.[35] Pyridoxine in low doses (10 mg/day) is also of therapeutic value for hyperoxaluric kidney stone formers.[36]

Animal studies have shown that B6 depletion leads to the development of hypertension, which is normalized within 24 hours by repletion with the vitamin.[37] There are several proposed mechanisms for this effect, but none that are proven.

There are many more proposed indications for P5P such as asthma, autism, acne, diabetes mellitus, depression, toxemia of pregnancy, and side effects of oral contraceptives, which are less clinically and experimentally documented.[38-44]

Safety, Toxicity, and Side Effects

The use of supplemental P5P has not been associated with toxicity, although the inactive form, pyridoxine, has been associated with reports of peripheral neuropathy.[45] One hypothesis is that pyridoxine toxicity is caused by exceeding the liver's ability to phosphorylate pyridoxine to P5P, yielding high serum levels of pyridoxine which may be directly neurotoxic or may compete with P5P for binding sites, resulting in a relative deficiency.[46]

Mpofu et al reported electrophysiological and neurological examination of 17 homocystinuric patients who had been treated with 200-500 mg pyridoxine HC1 daily for 10-24 years, and found no evidence of neuropathy.[47] Most reported cases of neuropathy associated with pyridoxine supplementation have involved intake of at least 500mg/day for two years or more.[48] While there is no doubt that vitamin B6 can be neurotoxic in gross excess, there is considerable controversy over the way in which toxicological data have been translated into advised limits.[8]

Drug/Nutrient Interactions

The antituberculosis drug isoniazid can result in a functional vitamin B6 deficiency.[49]

Anti-parkinsonian drugs benserazide and carbidopa cause vitamin B6 depletion by forming hydrazones.[50]

Pyridoxine will reduce the efficacy of levodopa in controlling parkinsonian symptoms, the magnitude of the effect proportional to the dose of pyridoxine.[51]

There have been many reports of abnormal tryptophan metabolism in women taking either oral contraceptive or menopausal hormone replacement therapy, which have been interpreted as indicating estrogen-induced vitamin B6 deficiency or depletion.[8]

Dosage

While the RDA for adults is 2-4 mg daily, the typical therapeutic dose is 50-200 mg/day.

References:

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[2.] Merrill AH Jr, Henderson JM. Vitamin B6 metabolism by human liven Ann N Y Acad Sci 1990;585:110-117.

[3.] Leklem JE. Vitamin B6:a status report. J Nutr 1990;120:1503-1507.

[4.] Haas E. Staying Healthy with Nutrition. The Complete Guide to Diet and Nutritional Medicine. Berkeley, CA: Celestial Arts Publishing;1992:122.

[5.] Parker TH, Marshall JP 2d, Roberts RK, et al. Effect of acute alcohol ingestion on plasma pyridoxal 5'-phosphate. Am J Clin Nutr 1979;32:1246-1252.

[6.] Labadarios D, Rossouw JE, McConnell JB, et al. Vitamin B6 deficiency in chronic liver disease -- evidence for increased degradation of pyridoxal-5-phosphate. Gut 1977;18:23-27.

[7.] Marz R. Medical Nutrition from Marz. A Textbook in Clinical Nutrition. Portland, OR: Omni Press;1997:211.

[8.] Bender D. Non-nutritional uses of vitamin B6. Br J Nutr 1999;81:7-20.

[9.] Mason DY, Emerson PM. Primary acquired sideroblastic anemia: response to treatment with pyridoxal-5-phosphate. Br Med J 1973;1:389-390.

[10.] Hines JD, Cowan DH. Studies on the pathogenesis of alcohol-induced sideroblastic bone-marrow abnormalities. N Engl J Med 1970;283:441-446.

[11.] Natta CL, Reynolds RD. Apparent vitamin B6 deficiency in sickle cell anemia. Am J Clin Nutr 1984;40:235-239.

[12.] Kark JA, Kale MP, Tarassoff PG, et al. Inhibition of erythrocyte sickling in vitro by pyridoxal. J Clin Invest 1978;62:888-891.

[13.] Kark JA, Tarassoff PG, Bongiovanni R. Pyridoxal phosphate as an antisickling agent in vitro. J Clin Invest 1983;71:1224-1229.

[14.] Fuhr JE, Farrow A, Nelson HS Jr. Vitamin B6 levels in patients with carpal tunnel syndrome. Arch Surg 1989;124:1329-1330.

[15.] Ellis JM. Treatment of carpal tunnel syndrome with vitamin B6. South Med J 1987;80:882-884.

[16.] Ellis J, Folkers K, Watanabe T, et al. Clinical results of a cross-over treatment with pyridoxine and placebo of the carpal tunnel syndrome. Am J Clin Nutr 1979;32:2040-2046.

[17.] Spooner GR, Desai HB, Angel JF, et al. Using pyridoxine to treat carpal tunnel syndrome. Randomized control trial. Can Fam Physician 1993;39:2122-2127.

[18.] Stransky M, Rubin A, Lava NS, Lazaro RP. Treatment of carpal tunnel syndrome with vitamin B6: a double blind study. South Med J 1989;82:841-842.

[19.] Franzblau A, Rock CL, Wemer RA, et al. The relationship of vitamin B6 status to median nerve function and carpal tunnel syndrome among active industrial workers. J Occup Environ Med 1996;38:485-491.

[20.] Ellis JM, Azuma J, Watanabe T, et al. Survey and new data on treatment with pyridoxine of patients having a clinical syndrome including the carpal tunnel and other defects. Res Commun Chem Pathol Pharmacol 1977;17:165-177.

[21.] Ellis JM, Folkers K. Clinical aspects of treatment of carpal tunnel syndrome with vitamin B6. Ann N YAcad Sci 1990;585:302-320.

[22.] Ellis JM, Folkers K, Levy M, et al. Response of vitamin B-6 deficiency and the carpal tunnel syndrome to pyridoxine. Proc Natl Acad Sci U S A 1982;79:7494-7498.

[23.] Ellis JM, Kishi T, Azuma J, Folkers K. Vitamin B6 deficiency in patients with a clinical syndrome including the carpal tunnel defect. Biochemical and clinical response to therapy with pyridoxine. Res Commun Chem Pathol Pharmacol 1976;13:743-757.

[24.] Jacobson MD, Plancher KD, Kleinman WB. Vitamin B6 (pyridoxine) therapy for carpal tunnel syndrome. Hand Clin 1996;12:253-257.

[25.] McCarty MF. High-dose pyridoxine as an `anti-stress' strategy. Med Hypotheses 2000;54:803-807.

[26.] Head KA. Premenstrual syndrome: nutritional and alternative approaches. Altern Med Rev 1997;2:12-25.

[27.] Wyatt KM, Dimmock PW, Jones PW, Shaughn O'Brien PM. Efficacy of vitamin B6 in the treatment of premenstrual syndrome: systematic review. BMJ 1999;318:1375-1381.

[28.] Selhub J, Jacques PF, Wilson PW, et al. Vitamin status and intake as primary determinants of homocysteinemia in an elderly population. JAMA 1993;270:2693-2698.

[29.] Rimm EB, Willett WC, Hu FB, et al. Folate and vitamin B6 from diet and supplements in relation to risk of coronary heart disease among women. JAMA 1998;279:359-364.

[30.] Van der Griend R, Haas FJ, Biesma DH, et al. Combination of low-dose folic acid and pyridoxine for treatment of hyperhomocysteinaemia in patients with premature arterial disease and their relatives. Atherosclerosis 1999;143:177-183.

[31.] Ubbink JB, Vermaak W J, van der Merwe A, et al. Vitamin requirements for the treatment of hyperhomocysteinemia in humans. J Nutr 1994;124:1927-1933.

[32.] Dierkes J, Kroesen M, Pietrzik K. Folic acid and Vitamin B6 supplementation and plasma homocysteine concentrations in healthy young women. Int J Vitam Nutr Res 1998;68:98-103.

[33.] Sahakian V, Rouse D, Sipes S, et al. Vitamin B6 is effective therapy for nausea and vomiting of pregnancy: a randomized, double-blind placebo-controlled study. Obstet Gynecol 1991;78:33-36.

[34.] Tsuge H, Hotta N, Hayakawa T. Effects of vitamin B-6 on (n-3) polyunsaturated fatty acid metabolism. J Nutr 2000;130:333S-334S.

[35.] Kojima R, Ito M, Suzuki Y. Studies on the nephrotoxicity of aminoglycoside antibiotics and protection from these effects (8): Protective effect of pyridoxal-5'-phosphate against tobramycin nephrotoxicity. Jpn J Pharmacol 1990;52:11-21.

[36.] Murthy MS, Farooqui S, Talwar HS, et al. Effect of pyridoxine supplementation on recurrent stone formers. Int J Clin Pharmacol Ther Toxicol 1982;20:434-437.

[37.] Dakshinamurti K, Lal KJ. Vitamins and hypertension. World Rev Nutr Diet 1992;69:40-73.

[38.] Sur S, Camara M, Buchmeier A, et al. Double-blind trial of pyridoxine (vitamin B6) in the treatment of steroiddependent asthma. Ann Allergy 1993;70:147-152.

[39.] Rimland B, Calloway E, Dreyfus P. The effect of high doses vitamin B6 on autistic children: a double-blind crossover study. Am J Psychiatry 1978;135:472-475.

[40.] Snider BL, Dieteman DF. Pyridoxine therapy for premenstrual acne flare. Arch Dermatol 1974;110:130-131. [Letter]

[41.] Spellacy WN, Buhi WC, Birk SA. Vitamin B6 treatment of gestational diabetes mellitus: studies of blood glucose and plasma insulin. Am J Obstet Gynecol 1977;127:599-602.

[42.] Russ CS. Vitamin B6 status of depressed and obsessive-compulsive patients. Nutr Rep Int 1983;27:867-873.

[43.] Wachstein G. Influence of B6 on the incidence of pre-eclampsia. Gynecol 1956;8:177.

[44.] Adams PW, Wynn V, Folkard J, et al. Influence of oral contraceptives, pyridoxine, and tryptophan on carbohydrate metabolism. Lancet 1975;1:759-764.

[45.] Schaumburg H, Kaplan J, Windebank A, et al. Sensory neuropathy from pyridoxine abuse. A new megavitamin syndrome. N Engl J Med 1983;309:445-448.

[46.] Parry GJ, Bredesen DE. Sensory neuropathy with low-dose pyridoxine. Neurology 1985;35:1466-1468.

[47.] Mpofu C, Alani SM, Whitehouse C, et al.. No sensory neuropathy during pyridoxine treatment in homocystinuria. Arch Dis Child 1991;66:1081-1082.

[48.] Bendich A, Cohen M. Vitamin B6 safety issues. Ann N Y Acad Sci 1990;585:321-330.

[49.] Standal BR, Kao-Chen SM, Yang GY, Char DF. Early changes in pyridoxine status of patients receiving isoniazid therapy. Am J Clin Nutr 1974;27:479-484.

[50.] Bender DA. Effects of benserazide, carbidopa and isoniazid administration on tryptophan-nicotinamide nucleotide metabolism in the rat. Biochem Pharmacol 1980;29:2099-2104.

[51.] Hunter KR, Stem GM, Laurence DR. Use of levodopa with other drugs. Lancet 1970;2:1283-1285.

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