A major mucus glycoprotein (mucin) was purified from the tracheobronchial secretions of an asthmatic patient. Upon SDS-composite gel electrophoresis, the purified native (non-reduced) mucin gave a single band. SDS-gel electrophoresis on 6% polyacrylamide gels showed the absence of low molecular mass protein contaminants. However, SDS-PAGE (6% gels) of the reduced mucin showed the presence of a major high molecular mass mucin component and two low molecular mass components of 118 and 70 kDa, respectively. The 118 and 70 kDa components were purified by preparative electroelution of the reduced mucin. These components were also separated from the reduced mucin by gel-permeation chromatography on a Superose 6 column. Chemical compositional analyses showed that the 118 kDa component was a glycoprotein while the 70 kDa component was non-glycosylated. The effect of disulfide bond reduction on mucin structure and the hydrophobic probe binding properties of native and reduced mucin were studied using the fluorescent probe technique. Mansylphenylalanine was used as the fluorescent probe. The native mucin showed the presence of a large number of low-affinity (KD approximately 10(-5) M) binding sites for the probe. On the other hand, reduced-alkylated mucin containing the 118 and 70 kDa components showed the presence of additional high-affinity (KD approximately 10(-6) M) binding sites as well as low-affinity binding sites for the probe. Reduced alkylated mucin devoid of the 118 and 70 kDa components showed the presence of only low-affinity binding sites. These observations suggest that the availability of high-affinity probe binding sites upon reduction of mucin disulfide bonds may be either due to binding of the probe to the released component(s) and/or due to noncovalent interaction of the released component(s) with the mucin causing a conformational change in the mucin structure. Thus, the 118 and 70 kDa components appear to be an integral part of the total polymeric structure of the human respiratory mucin.