Physicochemical and structural characterisation of protein isolate, globulin and albumin from soapnut seeds (Sapindus mukorossi Gaertn.)

The amino acid (AA) composition and physicochemical and conformational properties of protein isolate (SNPI), globulin (SNG) and albumin (SNA) fractions from soapnut seeds were evaluated. The essential AA of SNG, SNA and SNPI (except sulfur-containing AA) are sufficient for the FAO/WHO suggested requirements for 2–5 year old infants. SNG and SNPI showed similar electrophoresis patterns and AA compositions, the subunit of those proteins consisted of two polypeptides linked by disulfide bonds. In contrast, SNA showed a different AA compositions and SDS–PAGE pattern. Both SNG and SNPI presented a typical U-shape protein solubility (PS)–pH profile, SNA showed a completely different PS–pH profile, especially at pH 2.0–4.0. The near-UV circular dichroism (CD), differential scanning calorimetry (DSC) and tryptophan fluorescence spectra analyses indicated that the flexibility in tertiary conformations decreased in the order: SNA > SNPI > SNG, while soapnut proteins had a similar secondary conformation, with a highly ordered structure (the β-types), as evidenced by far-UV CD spectra.