Lysosomal digestion of thyroglobulin: role of cathepsin D and thiol proteases

Purified hog thyroid lysosomes, prepared by a procedure previously developed in this laboratory, were used to study lysosomal digestion of [131I]thyroglobulin [131I]Tg). The lysosomal proteases were solubilized with 0.1% Triton X-100. Rates of proteolytic digestion, measured by the release of ethanol-ammonium acetate-extractable 131I, were greatly stimulated by thiol reagents. The pH optimum was also affected by the presence of thiols. In the absence of a thiol reagent, a broad pH optimum was observed, ranging from 3.5-4.5. However, in the presence of 1 mM mercaptoethanol, the maximum rate of digestion occurred at pH 5.0, very close to reported values for the internal pH of lysosomes. Pepstatin, an inhibitor of cathepsin D, markedly inhibited lysosomal digestion of [131I]Tg at concentrations as low as 0.01 micrograms/ml. Its inhibitory effect was greater at pH 3.5 (pH optimum of cathepsin D) than at pH 5.0. Leupeptin, an inhibitor of thiol proteases, was not as potent as pepstatin, but it was significantly inhibitory at a concentration of 1 microgram/ml. In contrast to pepstatin, leupeptin displayed a greater inhibitory effect at pH 5.0 than at pH 3.5. The pH optimum of hog thiol proteases has been reported to range from 5.5-6.5. The effects of the two inhibitors were additive at pH 5.0. We conclude from these results that both cathepsin D and thiol proteases play a role in lysosomal digestion of Tg. Cathepsin D appears to be quantitatively more important than thiol protease in the initial phase of the digestion. The stimulatory effect of thiols on lysosomal digestion of [131I]Tg probably involves two separate effects: 1) stimulation of thiol proteases, and 2) reduction of S-S bonds in Tg, making the protein more susceptible to attack by proteolytic enzymes. Poorly iodinated [131I]Tg was more rapidly hydrolyzed than well iodinated [131I]Tg, based on the release of ethanol-ammonium acetate-extractable 131I. However, there was little or no difference in the rate of total peptide bond cleavage between poorly iodinated and well iodinated Tg. These results suggest that the first sites of iodination of Tg are preferentially attacked by lysosomal proteases. Long term (24-h) digestion of [131I]Tg with solubilized thyroid lysosomes at pH 5.0 in the presence of thiol compounds was just as effective as digestion with pronase at pH 8.0 in liberating free 131I-labeled iodothyronines and 131I-labeled iodotyrosines. Thus, thyroid lysosomes contain the full complement of proteases and peptidases required for cleaving free iodoamino acids from Tg.