including the increase in homocysteine after a methionine load, did not correlate as well.
Plasma PLP concentrations decrease slightly with increased protein intake. They are very high by comparison in the fetus, decrease fairly rapidly in the first year, and then decrease more gradually throughout the lifespan (Hamfelt and Tuvemo, 1972). It is not possible to evaluate whether the higher values in newborns and infants reflect ample body stores or whether the higher concentrations reflect normal status for this age group. Because of this, it is not possible to state that a 20 nmol/L concentration in the infant reflects a status equivalent to that for a 20 nmol/L PLP concentration in the adult.
Normally, plasma PLP is measured by using an apotyrosine decarboxylase assay. This assay has been well standardized and there is usually good interlaboratory agreement with it.
Erythrocyte and total blood PLP concentrations have also been used as measures of B6 status but not as extensively as plasma PLP. Erythrocyte PLP concentrations are similar to those for plasma PLP in individuals on normal diets, but they increase to much higher values than does plasma PLP in subjects taking large doses of the vitamin (Bhagavan et al., 1975). This reflects the high binding capacity of hemoglobin for PLP. Erythrocyte PLP is derived from plasma pyridoxal (PL); the erythrocyte contains PL kinase activity. Because of lower kinase activity, blacks may have lower erythrocyte PLP values than do whites. The small number of studies using erythrocyte values limits the ability to derive a concentration consistent with adequate status.
Blood concentrations of total vitamers of B6 as well as individual concentrations of specific B6 vitamers have been determined in some studies. These values tend to fluctuate considerably. They also fluctuate throughout the menstrual cycle, which limits their usefulness as status indicators (Contractor and Shane, 1968).
Urinary B6 excretion and 4-pyridoxic acid (4-PA) excretion have been used extensively to evaluate B6 requirements. Approximately