Increased plasma pyridoxal-5'-phosphate levels before and after pyridoxine loading in carriers of perinatal/infantile hypophosphatasia.

We measured plasma levels of pyridoxal-5'-phosphate (PLP), a cofactor form of vitamin B6 and apparent natural substrate for alkaline phosphatase (ALP), in carriers and in non-carriers of the severe perinatal and infantile forms of hypophosphatasia, both before and after an oral load of pyridoxine (i.e. 1/3 mg/kg body weight). The assignment of carrier status was determined by serum ALP activity, level of serum inorganic phosphate, and if necessary urinary phosphoethanolamine excretion. Plasma PLP levels were significantly increased in the carriers both before and especially after B6 loading.