Jmw Bouma

RAPID UPTAKE BY LIVER SINUSOIDAL CELLS OF SERUM-ALBUMIN MODIFIED WITH RETENTION OF ITS COMPACT CONFORMATION

The clearance from the blood and the conformation of serum albumin modified by nitroguanidination and labeled with 125-I have been studied. Like formaldehyde-denatured albumin, but in contrast to native albumin, the nitroguanidinated derivative is rapidly cleared from the blood and taken up in lysosomes of liver sinusoidal cells. Although 94% of the free amino groups were blocked by nitroguanidination, we could not detect significant conformational changes using gel filtration, determination of reducible disulfide groups, and titration of tyrosine residues. It is concluded that extensive denaturation is no prerequisite for the uptake of albumin derivatives in liver sinusoidal cells. It is suggested that the nitroguanidinated protein, in contrast to native albumin, is bound on membrane receptors of sinusoidal cells. The nitroguanidino groups themselves might be bound on these receptors, but it seems equally possible that the blocking of positive charges of the albumin molecule or minor, local conformational changes of the protein are sufficient for the binding on the receptors.

INDUCTION OF LYSOSOMAL STORAGE BY SURAMIN

SummaryIsolated livers of rats injected with saline or with suramin (250 mg per kg body weight) 24 h previously were perfused with a medium containing radioactively labeled formaldehyde-treated albumin. Suramin-loaded livers released breakdown products at a much lower rate than controls and contained about the double amount of undigested radioactive protein up to about 3 h after the start of the perfusion. These results show that inhibition of proteolysis by suramin as reported previously (Davies et al., 1971; Buys et al., 1973) is not caused by binding of the drug to the substrate in the bloodstream.Electron micrographs of liver sections of suramintreated rats showed that lysosomes of sinusoidal cells resembled those seen in certain lysosomal storage diseases.The effect of suramin on lysosomal enzymes was studied in vitro. When used at a concentration corresponding to the putative concentration in lysosomes in vivo, the drug inhibited the lysosomal endopeptidases cathepsin Bl and D as well as acid phosphatase. Inhibition of acid phosphatase by suramin in vivo could also be demonstrated by histochemical methods. These results suggest that the observed storage phenomena may be mainly caused by inhibition of lysosomal enzymes.