Plasma retention and metabolic fate of hemoglobin modified with an interdimeric covalent cross link

Hemoglobin (Hb) modified with an interdimeric bicovalent cross link using 2-nor-2-formylpyridoxal 5'-phosphate (NFPLP) as the cross-linking agent has an exceptionally low O2 affinity (P50 = 47 torr), enabling it to deliver more oxygen at tissue pO2 than blood. In addition, the covalent cross link prevents dissociation of the HbXL tetramers. By using 3H-labeled HbXL, the present study investigated intravascular retention time of cross linked Hb (HbXL), organ distribution, and routes by which HbXL is metabolized and eliminated from the body. The rats were injected with an i.v. bolus (125-200 mg Hb/kg body weight) of either 3H-labeled HbXL or noncross-linked pyridoxal 5'-phosphate modified Hb (diPLPHb) as a control. Urine and feces were collected daily for up to 9 days. Organs and tissues were harvested either at 9 hr or 9 days and assayed for 3H-label content by standard liquid scintillation counting. Plasma retention of HbXL at this dose was about three times longer than diPLPHb, and no HbXL as such was recovered in the urine. HbXL did undergo metabolic degradation in the body, with labeled fragments (mol. wt. less than 10,000) being excreted by the kidneys and gastrointestinal tract. Total body clearance of the label by 9 days amounted to approximately 83% of the injected dose.