Plasma clearance of 125I-labelled haemopexin in normal and haem-loaded rabbits.

Abstract

Summary. Plasma clearance of 125I‐labelled rabbit haemopexin has been studied in six normal control rabbits and in a test group of rabbits haem‐loaded with 25 mg of haem.Following intravenous injection, equilibration of 125I‐haemopexin between intravascular and extravascular compartments appeared to be complete within 24 hr and the decline in plasma activity then became constant in rate. The plasma activity curve was resolved into two exponential functions of time and from the data, fractional catabolic rate, intravascular:extravascular distribution of haemopexin and total haem binding capacity were calculated. Control group. Mean plasma half‐clearance time of haemopexin was 20.3 hr; fractional catabolic rate was 1.8 (SD 0.19) times the intravascular haemopexin pool per day. Extravascular haemopexin was 0.6 (SD 0.11) times the mass of haemopexin in the intravascular pool. Test group. 90–95% of injected 125I‐activity was eliminated from the plasma with a mean half‐clearance time of 1.57 hr and was associated with a significant urinary excretion of labelled iodine. Fractional catabolic rate was 5.84 (SD 0.3) times the intravascular haemopexin pool per day and extravascular haemopexin was 3.38 (SD 1.03) times the mass of haemopexin in the intravascular pool.Haemopexin concentration in normal rabbit plasma was 31–52 mg/100 ml; intravascular plus extravascular haemopexin was 54.5 mg (mean value) and total haem binding capacity of haemopexin was 0.53 mg (mean value).Intravascular haem rapidly complexed with haemopexin and the data suggested that this complex was promptly eliminated from the plasma and the haemopexin catabolized.