Extended [13C]galactose oxidation studies in patients with galactosemia

Since patients with galactose-1-phosphate uridyltransferase (GALT) deficiency have considerable endogenous galactose formation and only limited urinary excretion of galactose metabolites, there must be mechanisms for disposal of the sugar. Otherwise, a steady-state could not be maintained and there would be continuous body accumulation of galactose and alternate pathway products. Previous studies quantitating the amount of galactose handled by oxidation to CO2 focused on short collection periods of expired air after administering isotopically labeled galactose mainly designed for discerning differences in the capacity to oxidize the sugar in relation to genotype. Assuming that there may be more extensive oxidation than that observed in short-term studies in order to dispose the daily galactose burden, we have examined the amount of [1-13C]galactose oxidized to 13CO2 over a 24-h period after either a single bolus or continuous IV administration by 11 patients with classic galactosemia including patients homozygous for the Q188R gene mutation. As much as 58% of the administered galactose was oxidized to 13CO2 in 24 h. The pathways involved remain to be determined but a significant amount may be metabolized by non-GALT pathways since a patient homozygous for gene deletion had an oxidative capability. We conclude that classic patients have the ability to slowly oxidize galactose to CO2 in 24 h in amounts comparable to that which a normal handles in approximately one-fifth the time. This capacity enables the galactosemic to maintain a balance of galactose disposal with the galactose burden imposed by endogenous formation and dietary intake.