Calcium-dependent Golgi-vesicle fusion and cathepsin B in the conversion of proalbumin into albumin in rat liver

Abstract

1. An enzyme from rat liver that converts proalbumin into albumin is described. Partial purification, inhibitor studies and the conditions for maximum activity suggest that the enzyme is cathepsin B. 2. A membrane-bound enzyme, located mainly in lysosomes, also converts proalbumin into albumin. This appears to be a membrane-bound form of cathepsin B. 3. Isolated Golgi vesicles, incubated under conditions suitable for cathepsin B, convert endogenous proalbumin into albumin. 4. This conversion in Golgi vesicles has an absolute requirement for Ca2+ at micromolar concentrations. Mg2+ does not affect or substitute for Ca2+. Both the proalbumin and the albumin formed from it are intravesicular. 5. Converting activity is enhanced by pretreatment with the known chemical fusogen, poly(ethyleneglycol). 6. Vesicles preincubated at pH above 7 in the presence of dithiothreitol show a marked fall in converting activity. This can be partially restored by incubation with native vesicles. These results suggest that vesicle fusion is a requirement for conversion of proalbumin into albumin.

Intracellular localization of acid phosphatase and trypsinogen on an identical specimen in rat pancreas.

“…One of the roles of the lysosomes in secretory cells might be to degrade endogenous secretory products when the discharge of the products is depressed under certain physiological conditions (7,22). Ikehara et al (10) reported that proalbumin was converted to albumin in secretory vesicles of rat liver and almost the same results have reported by Quinn et al (19). Smith et al (23) suggested that acid protease included in immature secretory vesicles may also participate in the conversion of proinsulin to insulin in rat pacreas.…”

Weakly basic amines inhibit the proteolytic conversion of proalbumin to serum albumin in cultured rat hepatocytes

“…Chloroquine was reported to inhibit the cathepsin B activity in the crude homogenatc of fibroblasts [29]. Although cathepsin B was suggested to be a candidate for the processing enzyme for proalbumin [30], its involvement in the conversion of proinsulin and parathyroid hormone was not confirmed by the experiments using the purified cathepsin B [31, 321. In fact, addition of leupeptin (100 pgiml), a specific inhibitor of cathepsin B, exerted no inhibitory effect on the conversion of proalbumin in cultured hepatocytes (data not shown).…”

Calcium‐dependent KEX2‐like protease found in hepatic secretory vesicles converts proalbumin to albumin

“…Golgi vesicle fractions were prepared as described by Quinn and Judah [16]; a critical difference, however, was that no EDTA was incorporated in the various sucrose density solutions. Briefly, livers were homogenized in ice-cold 0.25 M sucrose; nuclei, mitoehondria and cell debris were removed by centrifugation (9500 x gay, 15 min).…”