Adsorptive pinocytosis of 125I-labelled lactate dehydrogenase isoenzymes H4 and M4 by rat yolk sacs incubated in vitro

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Abstract
The pinocytic uptake of 125I-labeled porcine lactate dehydrogenase isoenzymes H4 and M4 by 17.5-day rat visceral yolk sac incubated in vitro was saturable and binding obeyed Michaelis-Menten kinetics. The uptake characteristics of the 2 isoenzymes were very similar. For the H4 and M4 isoenzymes, the Kd of the protein-plasma-membrane complex were 0.62 and 0.84 .mu.M, respectively, and the maximum rates of uptake 0.13 and 0.26 nmol/mg of yolk sac protein/h, respectively. These findings contrast with those from studies in vivo, which show the M4 form is taken up by rat liver sinusoidal cells at a much higher rate than the H4 form and point to different recognition systems for the adsorptive pinocytosis of simple non-conjugate proteins in yolk-sac epithelial cells and liver sinusoidal cells. Competition experiments indicate that binding of the H4 isoenzyme to the yolk-sac cells is restricted to hydrophobic interactions; binding of the M4 isoenzyme involves hydrophobic and positively charged sites on the protein molecules.