The turnover of 125I-labelled lactoferrin was measured in 10 adults. Simultaneous organ radio-activity counting was performed. Ferrokinetic studies were performed in 3 adults after the intravenous injection of [59Fe]lactoferrin. Lactoferrin was rapidly eliminated from the plasma with a mean fractional catabolic rate of 5.7/day. Apolactoferrin (1 subject) was eliminated at a slower rate (fractional catabolic rate 1.22/day). Of the administered 125I label 99% was recovered in the urine, as free iodine, within the first 24 hr. In the 59Fe studies no appreciable activity was found in the urine. Organ radioactivity counting showed that lactoferrin was rapidly taken up by the liver and spleen. In the 125I studies the rapid excretion of free 125I suggested catabolism at these sites. In the 59Fe studies, the radioactivity persisted in the liver and spleen for several weeks and was slowly transferred to the bone marrow before appearing in circulating erythrocytes. From the values of fractional catabolic rate, plasma lactoferrin, neutrophil lactoferrin and plasma volume, a 'derived neutrophil turnover' was calculated for each subject. The mean value was 8 x 108 neutrophils/day. This is about 1% the value obtained from the actual measurement of labelled cells. It is postulated that this 'derived value' represents only that portion of neutrophil turnover accounted for by intravascular senescence.
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