The plasma half clearance time (T1/2) of isotope-labeled rabbit hemopexin was 35.5 plus or minus 1.9 hr in rabbits. After intra-and extra-vascular equilibration of 125I-hemopexin and 131I-albumin, injection of either hematin, hemoglobin or methemoglobin (12.5 mg of heme/kg body wt) resulted in the rapid removal of 60-80% of circulating hemopexin (T1/2 = 7.2 plus or minus 0.6 hr) but did not affect albumin catabolism. After selective photo-inactivation of hemopexin, the T1/2 of this hemopexin was comparable to that of the native molecule. However, its plasma disappearance curve was not appreciably affected by administration of hematin. These findings demonstrate that hemopexin is cleared and catabolized at an enhanced rate during states of plasma heme load, and that modification of critical histidine residues of hemopexin eliminates its biological function in plasma heme disposal.
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