Kinetics of Human Serum Amyloid A

Abstract

In order to better understand the pathogenetic role of serum amyloid A (SAA) we studied the kinetics of ¹³¹I radiolabelled pure SAA, extracted from 400 ml serum of a human volunteer. 50 μCi of ¹³¹I SAA and 15 μCi ¹²⁵I labelled sodium iodide were administered i.v. on two occasions at 6 month intervals. Serum and plasma samples were collected at 10–20 min intervals × 10, then once daily × 10; lymphocytes were separated from monocytes and granulocytes. Counts per minute of ¹³¹I and ¹²⁵I were measured in each sample in the serum, in serum precipitates resulting after addition of a rabbit anti-SAA antibody and of TCA and in various cell subpopulations as well as in the whole urine and TCA precipitated urine from each micturition. The ¹³¹I disappearance curves from the plasma and serum precipitates were semilogarithmically plotted; cumulative ¹³¹I cpm in plasma, cells and urine at various intervals were determined. Body scanning was performed at 2, 16, and 48 h. The results of the two experiments were very similar. The curve of ¹³¹SAA in plasma TCA precipitates indicated the existence of 4 compartments likely due to uptake of ¹³¹I SAA by some plasma proteins, circulating cells and other tissues; later release from tissues started at 6 h. The ¹³¹I SAA half-life time in these compartments was found to be 35, 170, 255, and 550 min, respectively. Tissue binding of ¹³¹I was also suggested by a rising of the ¹²⁵I:¹³¹I ratio with time and by a 26% release of ¹³¹I in the urine at 15 h which could not account for its plasma disappearance. Scanning, except for ¹³¹I uptake in the spleen at 2 h likely due to blood activity, showed no organ concentration. 92% of the injected ¹³¹I was found in the urine but only 6.2% of ¹³¹ I SAA was accounted for in urine precicipitates.