J. Biol. Chem., Vol. 263, Issue 28, 14122-14127, Oct, 1988
Inulin-125I-tyramine, an improved residualizing label for studies on sites of catabolism of circulating proteins
JL Maxwell, JW Baynes and SR Thorpe
Department of Chemistry, University of South Carolina, Columbia 29208.
Residualizing labels for protein, such as dilactitol-125I-tyramine
(125I-DLT) and cellobiitol-125I-tyramine, have been used to identify the
tissue and cellular sites of catabolism of long-lived plasma proteins, such
as albumin, immunoglobulins, and lipoproteins. The radioactive degradation
products formed from labeled proteins are relatively large, hydrophilic,
resistant to lysosomal hydrolases, and accumulate in lysosomes in the cells
involved in degradation of the carrier protein. However, the gradual loss
of the catabolites from cells (t1/2 approximately 2 days) has limited the
usefulness of residualizing labels in studies on longer lived proteins. We
describe here a higher molecular weight (Mr approximately 5000), more
efficient residualizing glycoconjugate label, inulin-125I-tyramine
(125I-InTn). Attachment of 125I-InTn had no effect on the plasma half-life
or tissue sites of catabolism of asialofetuin, fetuin, or rat serum albumin
in the rat. The half-life for hepatic retention of degradation products
from 125I-InTn-labeled asialofetuin was 5 days, compared to 2.3 days for
125I-DLT-labeled asialofetuin. The whole body half-lives for radioactivity
from 125I-InTn-, 125I-DLT-, and 125I-labeled rat serum albumin were 7.5,
4.3, and 2.2 days, respectively. The tissue distribution of degradation
products from 125I-InTn-labeled proteins agreed with results of previous
studies using 125I-DLT, except that a greater fraction of total degradation
products was recovered in tissues. Kinetic analyses indicated that the
average half-life for retention of 125I-InTn degradation products in
tissues is approximately 5 days and suggested that in vivo there are both
slow and rapid routes for release of degradation products from cells.
Overall, these experiments indicate that 125I-InTn should provide greater
sensitivity and more accurate quantitative information on the sites of
catabolism of long-lived circulating proteins in vivo.
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