Vitamin B6 Dependency Syndromes

A small number of patients show one or the other of the biochemical signs associated with vitamin B6 deficiency despite apparently adequate status, and require high intakes of the vitamin to normalize the abnormal metabolic marker. These are genetic diseases and have been termed vitamin B6 dependency syndromes.

As shown in Table 9.4, vitamin B6 dependency has been reported in cases of type I primary hyperoxaluria, xanthurenic aciduria, homocystinuria, hypochromic sideroblastic anemia, gyrate atrophy with ornithinemia, and vitamin B6 responsive infantile convulsions. In this last condition, the underlying defect has not been identified, but is almost certainly not impaired activity of glutamate decarboxylase.

The molecular basis of the other vitamin B6 dependency syndromes is a severely reduced affinity of the defective enzyme for its cofactor, and the patients respond well to doses of 50 to 1,000 mg of vitamin B6 per day. Apart from the affected enzyme, other biochemical indices of vitamin B6 nutritional status are normal in these patients (Frimpter et al., 1969; Mudd, 1971).

9.5 THE ASSESSMENT OF VITAMIN Be NUTRITIONAL STATUS

As shown in Table 9.5, there are a number of indices of vitamin B6 status available: plasma concentrations of the vitamin, urinary excretion of 4-pyridoxic acid, activation of erythrocyte aminotransferases by pyridoxal phosphate added in vitro, and the ability to metabolize test doses of trypto-phan and methionine. None is wholly satisfactory; and where more than one index has been used in population studies, there is poor agreement between the different methods (Bender, 1989b; Bates et al., 1999a).

Table 9.5 Indices of Vitamin B6 Nutritional Status

Adequate Status

Plasma total vitamin B6

>40 nmol (10 |xg)/L

Plasma pyridoxal phosphate

>30 nmol (7.5 |g)/L

Erythrocyte alanine aminotransferase activation coefficient

<1.25

Erythrocyte aspartate aminotransferase activation coefficient

<1.80

Erythrocyte aspartate aminotransferase

>0.13 units (8.4 |kat)/L

Urine 4-pyridoxic acid

>3.0 |imol/24 h

>1.3 mmol/mol creatinine

Urine total vitamin B6

>0.5 |imol/24 h

>0.2 mmol/mol creatinine

Urine xanthurenic acid after 2 g tryptophan load

<65 |xmol/24 h increase

Urine cystathionine after 3 g methionine load

<350 | mol/24 h increase

Sources: From data reported by McChrisley et al., 1988; Leklem, 1990; Bitsch, 1993.

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