BACKGROUND: B-type natriuretic peptide (BNP) compensates for the failing heart and is synthesized as a 108-residue prohormone that is cleaved to a 32-residue C-terminal maximally active peptide. During heart failure, serum concentrations of proBNP 1-108 exceed concentrations of BNP 1-32. The aim of this study was to determine why the proBNP 1-108/BNP 1-32 ratio increases and whether proBNP 1-108 is bioactive. METHODS: Using cGMP elevation and 125I-ANP binding assays, we measured binding and activation of individual human natriuretic peptide receptor populations by recombinant human proBNP 1-108 and human synthetic BNP 1-32. Using receptor bioassays, we measured degradation of recombinant proBNP 1-108 and BNP 1-32 by human kidney membranes. RESULTS: ProBNP 1-108 stimulated guanylyl cyclase-A (GC-A) to near-maximum activities but was 13-fold less potent than BNP 1-32. ProBNP 1-108 bound human GC-A 35-fold less tightly than BNP 1-32. Neither proBNP 1-108 nor BNP 1-32 activated GC-B. The natriuretic peptide clearance receptor bound proBNP 1-108 3-fold less tightly than BNP 1-32. The half time for degradation of proBNP 1-108 by human kidney membranes was 2.7-fold longer than for BNP 1-32, and the time required for complete degradation was 6-fold longer. BNP 1-32 and proBNP 1-108 were best fitted by first- and second-order exponential decay models, respectively. CONCLUSIONS: ProBNP 1-108 activates GC-A with reduced potency and is resistant to degradation. Reduced degradation of proBNP 1-108 may contribute to the increased ratio of serum proBNP 1-108 to BNP 1-32 observed in patients with congestive heart failure.