The pinocytic uptake of 125I-labelled 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 two isoenzymes were very similar. For the H4 and the M4 isoenzymes, the dissociation constants of the protein-plasma-membrane complex were 0.62 microM and 0.84 microM respectively, and the maximum rates of uptake 0.13 and 0.26 nmol/mg of yolk-sac protein per 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, whereas the binding of the M4 isoenzyme involves hydrophobic as well as positively charged sites on the protein molecules.

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