Rates of disappearance from plasma of enzymes labeled by coupling with a radioactive lodo-ester.
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 intravenously into rabbits and 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 intravenous injection of creatine kinase labeled similarly. We conclude that this method of labeling is unsuitable for preparing radioactive enzymes for study of their catabolism.
D000818:Animals; D003402:Creatine Kinase / Q000097:blood*; D006207:Half-Life; D007457:Iodine Radioisotopes; D007527:Isoenzymes; D007553:Isotope Labeling / Q000379:methods; D007700:Kinetics; D007770:L-Lactate Dehydrogenase / Q000097:blood*; D010666:Phenylpropionates; D011817:Rabbits; D013388:Succinimides
D007457:Iodine Radioisotopes; D007527:Isoenzymes; D010666:Phenylpropionates; D013388:Succinimides; D007770:L-Lactate Dehydrogenase; D003402:Creatine Kinase