Hereditary deficiency of the 79 amino acid SP-B peptide in human infants and SP-B knockout mice invariably results in neonatal respiratory distress syndrome and death. Although the mature airway form of SP-B is a sulfhydryl-dependent dimer (Mr approx. 18 kD), the role of dimer formation in SP-B function is unknown. The current study was undertaken to test the hypothesis that dimerization of SP-B is essential for proper function, structure, and metabolism of pulmonary surfactant. To test this hypothesis, a dimerization deficient human SP-B construct was generated by mutation of the cysteine that is purported to be critical for dimer formation (cys48 --> ser). The sequence encoding mutated SP-B preproprotein was subsequently cloned under the control of the 3.7 kB human SP-C promoter, which directs expression to type II and Clara cells, and injected into fertilized FVB/N eggs. Three transgenic lines were established and characterized. Monomeric, mature human SP-B peptide was detected in lung homogenates from transgene positive animals following SDS-PAGE under non-reducing conditions. The level of transgene expression was comparable to endogenous murine SP-B peptide levels in all three transgenic lines and resulted in no overt pathophysiology; further, human SP-B monomers were detected in alveolar lavage fluid of transgene positive animals, indicating that the monomeric form was secreted. We conclude that dimerization of SP-B is not essential for correct targeting of SP-B to the lamellar body or for proteolytic processing of the SP-B proprotein to the mature peptide. These results suggest that dimer formation is not critical for intracellular trafficking of SP-B. In order to test the hypothesis that dimerization is essential for the extracellular function of SP-B, dimerization deficient transgenic mice are currently being crossed with hemizygous mice(SP-B +/-) in order to achieve expression of human monomeric SP-B in SP-B knockout mice. These mice will be assessed to determine if monomeric SP-B can rescue the lethal SP-B(-/-) phenotype and to evaluate the effect of monomeric SP-B expression on lung compliance, alveolar epithelial cell ultrastructure, and surfactant phospholipid composition. (Supported by NIH HL56285 to TEW).