Holt C, Timmins P A, Errington N, Leaver J
Hannah Research Institute, Ayr, UK.
Eur J Biochem. 1998 Feb 15;252(1):73-8. doi: 10.1046/j.1432-1327.1998.2520073.x.
Calcium phosphate nanoclusters were prepared under standardised conditions using 10 mg ml(-1) of the 25-amino-acid N-terminal tryptic phosphopeptide of bovine beta-casein as a stabilising agent. The Mr determined by sedimentation equilibrium was 197,600+/-13,700 and the apparent radius of gyration determined by X-ray scattering was 2.80+/-0.05 nm. A small-angle neutron scattering contrast variation study in 1H2O/2H2O mixtures was performed and gave radii of gyration at the calculated match points for the calcium phosphate (88.2% 2H2O) and phosphopeptide (41.3% 2H2O) of 3.39+/-0.08 nm and 1.85+/-0.05 nm, respectively. Measurements at larger scattering wave vector showed a subsidiary maximum at about Q = 1.6 nm(-1). The results are consistent with a model of the nanoclusters comprising a spherical core of 355+/-20 CaHPO4 x 2 H2O units, density 2.31 g ml(-1) and radius 2.30+/-0.05 nm surrounded by 49+/-4 peptide chains with a partial specific volume of 0.7 cm3 g(-1), forming a tightly packed shell with an outer radius of 4.04+/-0.15 nm. This model suggests that the phosphopeptide is able to arrest the process of growth of the precipitating phase of calcium phosphate at its earliest stages. A similar role for whole casein could be vital to the normal functioning of the mammary gland during milk secretion.
在标准化条件下，以10 mg/ml的牛β-酪蛋白25个氨基酸的N端胰蛋白酶磷酸肽作为稳定剂制备磷酸钙纳米团簇。通过沉降平衡测定的相对分子质量为197,600±13,700，通过X射线散射测定的表观旋转半径为2.80±0.05 nm。在1H2O/2H2O混合物中进行了小角中子散射对比变化研究，得出磷酸钙（88.2% 2H2O）和磷酸肽（41.3% 2H2O）在计算出的匹配点处的旋转半径分别为3.39±0.08 nm和1.85±0.05 nm。在较大散射波矢处的测量显示在约Q = 1.6 nm-1处有一个次级最大值。结果与纳米团簇模型一致，该模型包括一个由355±20个CaHPO4·2H2O单元组成的球形核心，密度为2.31 g/ml，半径为2.30±0.05 nm，周围环绕着49±4条肽链，偏比容为0.7 cm3/g，形成一个紧密堆积的壳，外半径为4.04±0.15 nm。该模型表明，磷酸肽能够在磷酸钙沉淀阶段的最早阶段阻止其生长过程。全酪蛋白的类似作用可能对乳腺在乳汁分泌过程中的正常功能至关重要。
