Distribution of antithrombin III in rabbits: role of host and protein

Unlike in the case of some other species, the plasma curve of iodine-labeled antithrombin III (I-AT-III) in rabbits requires fitting with a three-term exponential function for obtaining reliable estimates of the catabolic rate and distribution of I-AT-III among various body compartments (Carlson, Atencio, and Simon. J. Clin. Invest. 74: 191-199, 1984). To decide whether this phenomenon is referable to the host or the protein, the behavior of rabbit and human I-AT-III was comparatively analyzed in rabbits. Data obtained with rabbit I-AT-III confirmed the findings by Carlson and co-workers. Human I-AT-III assumed a distribution that closely paralleled that of homologous I-AT-III, thus suggesting that the pattern of distribution is determined by the host species rather than its AT-III. Rabbits metabolized human I-AT-III 1.61 times faster than homologous I-AT-III by an unknown mechanism not involving immune response; a facet that may prove useful for the identification of the sites of catabolism of AT-III. The exponent of the body weight was calculated for the relationship between species size and AT-III turnover. A value of 0.5 was obtained that is distinctly lower than the exponents found earlier for some other plasma proteins.