AJP - Regulatory, Integrative and Comparative Physiology, Vol 253, Issue 3 383-R388, Copyright © 1987 by American Physiological Society
Why do total-body decay curves of iodine-labeled proteins begin with a delay?
E. Regoeczi
The initial delay that occurs in total-body radiation curves reaching their
single-exponential slopes was analyzed from 106 experiments involving
several mammalian species (guinea pig, mouse, rabbit, and rat) and plasma
proteins (alpha 1-acid glycoprotein, antithrombin III, fibrinogen,
immunoglobulin G, and transferrin) in 14 different combinations. The time
interval (Td) between injection and the intercept of the slope with the
full-dose value was adopted as a measure of curve nonideality. The overall
mean Td was 6.6 h, but individual values showed a significant correlation
to protein half-lives, whereby proteins of unequal metabolic properties
exhibited different mean Td values. Targeting protein to the liver
abolished delay. Choice of the isotope (125I or 131I) and size of the
labeled protein had no influence on the magnitude of delay. Whole-body
radiation curves of animals that received [125I]iodotyrosines, Na131I, or
131I-polyvinylpyrrolidone exhibited no initial delays. These results do not
support the earlier notion that delay is caused by a redistribution of the
labeled protein in the body to radiometrically more favorable sites.
However, they are compatible with the assumption that delayed passage of a
protein dose through the extracellular matrix and/or retarded transfer of
proteolytic products from extravascular catabolic sites to plasma may be
responsible for the phenomenon.
