MOLECULAR DETERMINANTS IN THE PLASMA-CLEARANCE AND TISSUE DISTRIBUTION OF RIBONUCLEASES OF THE RIBONUCLEASE-A SUPERFAMILY

The similarities and differences among members of the RNase A superfam ily provide an ideal opportunity to examine the molecular basis for di fferences in their pharmacokinetics and biodistribution. Plasma cleara nces in BALB/c mice are similar among the five RNases studied: human p ancreatic RNase, angiogenin, eosinophil-derived neurotoxin, onconase, and bovine seminal RNase. The average clearance is 0.13 ml/min or 60% of the glomerular filtration rate (measured by [C-14]inulin clearance during continuous infusion from an i.p. implanted osmotic pump). Angio genin has a higher volume of distribution and plasma-to-muscle transpo rt rate than the other RNases, suggestive of binding to endothelial ce lls. Organ distribution differs dramatically among these RNases. The R Nase most toxic to tumor cells, onconase, exhibits the longest retenti on in the kidneys: at 180 min, 50% of the injected dose is found in th e kidneys, whereas only 1% or less of the other RNases is retained in the kidneys. Slower elimination of onconase from the kidneys may be du e to a higher degree of binding in the kidney or a resistance to prote olytic degradation. To elucidate the molecular determinants involved i n tissue uptake, we examined the biodistribution of recombinant oncona se and two onconase-pancreatic RNase chimeric proteins. The tissue ret ention property of onconase appears to be located in at least two regi ons, one of which is in the NH2-terminal 9-amino acid alpha-helix. The NH2-terminal pyroglutamate of onconase, a residue essential for ribon ucleolytic activity and cytotoxicity, does not play a role in kidney r etention.