Rates of disappearance from plasma of enzymes labeled by coupling with a radioactive lodo-ester
Qureshi, A.R.; Wilkinson, J.H.
Clinical Chemistry 22(8): 1277-1282
1976
ISSN/ISBN: 0009-9147 PMID: 949836 Document Number: 108100
Lactate dehydrogenase-5 and creatine kinase from rabbit muscle were labeled by coupling with N-hydroxysuccinimidyl 3-(4'-hydroxy-[3',5'-125I]diiodophenyl)propionate. After purification, the analytical recovery of catalytically-active labeled enzyme averaged 90% for lactate dehydrogenase, 81% for creatine kinase. The labeled enzymes were injected i.v. into rabbits; disappearance from plasma of catalytic activity and radioactivity was measured. The disappearance curves for lactate dehydrogenase-5 differed considerably from those observed with the enzyme labeled by direct iodination. The discrepancy was due to rapid hydrolysis in vivo of the labeled amide-enzyme linkage, because about 50% of the injected radioactivity appeared in the urine as 125I-labeled 3-(4'-hydroxy-3',5'-diiodophenyl)propionic acid within 4-8 h of injection. Similar outputs were observed after administration of this acid to rabbits. The free acid was also detected in the urines of rabbits within 4-8 h of the i.v. injection of creatine kinase labeled similarly. This method of labeling is unsuitable for preparing radioactive enzymes for study of their catabolism.