RADIOCHEMICAL ASSAY OF MINUTE QUANTITIES OF GALACTOSE-1-PHOSPHATE URIDYLTRANSFERASE ACTIVITY IN ERYTHROCYTES AND LEUKOCYTES OF GALACTOSEMIAPATIENTS

A sensitive radioisotopic method has been developed which can detect g alactose-1-phosphate uridyltransferase (GALT) activity as low as 0.1% of normal control values in both erythrocytes and leukocytes. This ass ay utilizes carbon-14 labeled galactose-1-phosphate with high specific activity and requires removal of endogenous galactose-1-phosphate (Ga l-1-P) and uridine diphosphate glucose (UDPGlc) through dialysis. Opti mal exogenous UDPGlc concentration has been determined with a fixed co ncentration of Gal-1-P in the incubation. The rate of product, uridine diphosphate galactose (UDPGal), formation is monitored at three diffe rent times. Among 423 patients with galactosemia studied by this metho d, 363 patients exhibited no detectable GALT activity in their erythro cytes and 60 patients were found to have detectable erythrocyte GALT a ctivity ranging from 0.02 to 5.0 units (normal values: >20 units), The former group of patients was designated as classic galactosemia (GG) and the latter group as galactosemia variant (GV). Leucocytes from ten patients belonging to the GG group also showed complete absence of GA LT activity while leukocytes from two patients belonging to the GV gro up showed GALT activity at levels comparable with those found in their erythrocytes, Because there is extensive biochemical heterogeneity am ong galactosemia patients, we recommend that an assay with increased s ensitivity be carried out on blood samples from galactosemia patients so that clinical, biochemical and molecular correlations made by diffe rent groups of investigators can be compared.