A new protease in hog thyroid lysosomes. I. The presence of a leupeptin-sensitive protease in the soluble fraction of thyroid lysosomes.

The presence of protease activity at pH 7.5 and at acidic pH was demonstrated in the soluble fraction of lysosomes which were prepared from hog thyroid homogenates by subcellular fractionation. The ratios of specific activity measured at pH 7.5 to that at pH 3.5 were 0.07 to 0.08 for casein (at pH 7.5) or haemoglobin (at pH 3.5) as substrate and 0.13 for iodoamino acid-releasing activity from thyroglobulin as substrate. The protease activity measured at pH 7.5 was inhibited by inhibitors for neutral type protease, such as leupeptin, and by sulfhydryl inhibitors, such as iodoacetamide, indicating that the protease was a leupeptin-sensitive protease and contained a sulfhydryl group for its active site. Insensitiveness of the protease activity to di-isopropyl fluorophosphate excluded the involvement of a serine group at the active site. At pH 5.5 the proteolytic and iodoamino acid-releasing activities using casein and thyroglobulin, respectively, were also investigated. Several lines of evidence suggested that both activities measured at pH 5.5 were due to a mixture of acidic and leupeptin-sensitive protease activities.