The effect of chemical treatments of albumin and orosomucoid on rate of clearance from the rat bloodstream and rate of pinocytic capture of rat yolk sac cultured in vitro

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Abstract
Portions of a 125I-iodinated bovine serum albumin preparation were exposed to freezing, acetic acid (pH 3.5, 3.0 or 2.5), urea or formaldehyde, and the effect of these treatments on the rates of pinocytic uptake by yolk sacs from 17.5-day-pregnant rats cultured in vitro and of clearance from the rat bloodstream were studied. Uptake of albumin by the yolk sac was followed by rapid release of [125I]iodo-L-tyrosine into the culture medium. Clearance of albumin in vivo was accompanied by the appearance of trichloroacetic acid-soluble radioactivity in the bloodstream. In both systems the uptake rates of modified albumin preparations formed a series: formaldehyde or urea > acetic acid > freezing. The increased uptake rates of modified albumin preparations could not be ascribed to the formation of aggregates nor, in the yolk-sac system, to an increase in the pinosome formation rate. The various treatments, to which the albumin was subjected increase to varying degrees the affinity of the albumin molecule or binding sites on that region of the plasma membrane from which pinocytic vesicles are formed. Some comparable experiments with native and desialylated human orosomucoid indicate that the rat yolk-sac epithelial cells do not possess the recognition system for uptake of asialoglycoproteins that exists on the surface of hepatic parenchymal cells.