Cell association and degradation of pregnancy zone protein-chymotrypsin complex in cultured human monocytes

Abstract

¹²⁵I-labelled pregnancy zone protein complexed with chymotrypsin (PZP-ct) bound to freshly isolated and to cultured human monocytes. Binding of uncomplexed ¹²⁵I-pregnancy zone protein (PZP) was less than 20% of complex binding. At 4°C, labelled PZP-ct complex bound to cultured monocytes with a half-time of about 4h. Dissociation at 4°C was slow at low receptor occupancies (t1/2°24 h). At high receptor occupancies, dissociation was biphasic (initial k_-1=8°10⁻³ min⁻¹) and 85% of the cell-associated label had dissociated within 24 h. At near equilibrium, binding of 20 pmol/l ¹²⁵I-PZP-ct was half maximally inhibited at a ligand concentration of about 450 pmol/l. The Scatchard plot was linear giving an estimated concentration of 2×10⁴ receptors/cell. Ligand bound at 4°C was rapidly internalized at 37°C (half-time=0.5 min) and after a 5-10 min lag time, radioactivity comprising monoiodotyrosine was released into the medium following a sigmoidal curve. At 37°C the uptake of ¹²⁵I-PZP-ct complex was initially linear and within 15 min internalized label accumulated, reaching a steady state at about 85% of the cell-associated radioactivity. Following a lag time of 15 min, radioactivity soluble in trichloroacetic acid appeared in the medium. Similar results were obtained with ¹²⁵I-α₂-macroglobulin-trypsin complex (α₂M-T). It is concluded that high affinity receptors mediate binding, uptake and degradation of pregnancy zone protein—and α₂-macroglobulin-proteinase complex in human monocyte-macrophages.

Key Words:

• α₂-macroglobulin receptor
• cell association
• pregnancy zone protein-chymotrypsin complex
• protein degradation