A 50-kDa variant form of human surfactant protein D

The dominant form of human surfactant protein D (SP-D) is a multimeric coll agenous glycoprotein composed of monomeric subunits that have a molecular m ass of 43 kDa under reducing conditions, However, in evaluating monoclonal antibodies to human SP-D, an additional monomeric subunit was identified wi th a reduced molecular mass of 50 kDa. This 50-kDa variant was detected in approximately half of the samples evalu ated and was found in lavage fluid from normal subjects, patients with alve olar proteinosis or idiopathic pulmonary fibrosis and in amniotic fluid. Th is 50-kDa variant had the same amino-terminal sequence, amino acid composit ion and apparent size of the carboxy-terminal collagenase-resistant fragmen t (20 kDa) as the 43-kDa subunit. The major difference was in the amino-ter minal portion of the molecule and was due to altered glycosylation, as dete rmined by carbohydrate staining, chemical deglycosylation, treatment with N -glycanase and neuraminidase and reduced signals for threonine at positions 5, 9 and 10 during amino-terminal sequencing. After gel filtration chromatography, the 50-kDa form was not present in the high molecular weight fraction, which is commonly used in purification of SP-P), but was found only in the smaller molecular weight fraction of monom ers and trimers of SP-D. In conclusion, the 50 kDa-form of surfactant protein D is produced by post- translational glycosylation and does not form higher ordered oligomers, but its precise physiological function remains to be determined.