title: Sweetness characterization of recombinant human lysozyme.
abstract: Lysozyme, a bacteriolytic enzyme, is widely distributed in nature and is a component of the innate immune system. It is established that chicken egg lysozyme elicits sweetness. However, the sweetness of human milk lysozyme, which is vital for combating microbial infections of the gastrointestinal tract of breast-fed infants, has not been characterized. This study aimed to assess the elicitation of sweetness using recombinant mammalian lysozymes expressed in Pichia pastoris. Recombinant human lysozyme (h-LZ) and other mammalian lysozymes of mouse, dog, cat and bovine milk elicited similar sweetness as determined using a sensory test, whereas bovine stomach lysozyme (bs-LZ) did not. Assays of cell cultures showed that h-LZ activated the human sweet taste receptor hT1R2/hT1R3, whereas bs-LZ did not. Point mutations confirmed that the sweetness of h-LZ was independent of enzyme activity and substrate-binding sites, although acidic amino acid residues of bs-LZ played a significant role in diminishing sweetness. Therefore, we conclude that elicitation of sweetness is a ubiquitous function among all lysozymes including mammalian lysozymes. These findings may provide novel insights into the biological implications of T1R2/T1R3-activation by mammalian lysozyme in the oral cavity and gastrointestinal tract. However, the function of lysozyme within species lacking the functional sweet taste receptor gene, such as cat, is currently unknown.
authors.affiliation: Department of Fermentation Science, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan.
authors.forename: Mami
authors.initials: M
authors.lastname: Matano
authors.affiliation: Department of Fermentation Science, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan.
authors.forename: Kana
authors.initials: K
authors.lastname: Nakajima
authors.affiliation: Department of Fermentation Science, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan.
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Kashiwagi
authors.affiliation: Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
authors.forename: Shigezo
authors.initials: S
authors.lastname: Udaka
authors.affiliation: Department of Fermentation Science, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan. Electronic address: maehashi@nodai.ac.jp.
authors.forename: Kenji
authors.initials: K
authors.lastname: Maehashi
mesh_terms: D004789:Enzyme Activation; D006801:Humans; D009113:Muramidase / Q000737:chemistry*; D010843:Pichia / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry*; D013649:Taste
chemical_list: D011994:Recombinant Proteins; D009113:Muramidase
keywords: Human lysozyme; Milk; Pichia pastoris; Sweet taste receptor; Sweetness