Human Splenic Galaptin: Physicochemical Characterization

Mammalian spleens were previously reported to contain β-galactoside-binding lectins [Allen, H. J., Cywinski, M., Palmberg, R., & DiCioccio, R. (1987) Arch. Biochem. Biophys. 256, 523–533]. The aim of the present investigation was to determine the relationship of human splenic galaptin to other β-galactoside-binding lectins identified in other human and animal tissues. Galaptin of subunit molecular mass 14.5 kDa was the only lectin of this type found in human spleen as assessed by SDS-PAGE, RP-HPLC, and Western blot analyses. Three polypeptides of pI 4.60, 4.80, and 4.85 were detected by isoelectric focusing of purified galaptin, with the major band having pI 4.85. UV spectral analysis indicated the absence of prosthetic groups and gave A₁^1%_{cm, 280} = 5.5. Circular dichroic analysis suggested the presence of 40% β structure, considerable random coil, and 10% α helix structure. The amino acid composition was very similar to that for human placental galaptin. Amino acid sequence analyses were carried out on V8 protease, CNBr, and iodosobenzoic acid digestion fragments. A total of 94 residues were identified. All sequences determined could be aligned with placental galaptin sequences. We conclude that human splenic galaptin is identical with human placental galaptin. A related polypeptide of molecular mass ~14.5 kDa was found to be present in several different mammalian spleens as assessed by Western blot analysis using a monospecific polyclonal anti-human splenic galaptin antiserum.