Shunrokuro Arima

A sedimentation equilibrium study of the temperature-dependent association of bovine β-casein

The temperature-dependent association of beta-casein was studied by the sedimentation equilibrium method. The weight-average molecular weight of the protein was determined in 0.2 M sodium phosphate buffer (pH 6.7) at 20, 15, 10 and 2 degrees C, and was found to be dependent markedly on both temperature and protein concentration. The data were well fitted by a monomer-n-mer association scheme, and the values of n, the second viral coefficient, and the free energy change for the association (association constant) were evaluated. The results show that temperature not only shifts the equilibrium but alters the polymer size: the values of n are 49, 22, and 12 at 20, 15, and 10 degrees C, respectively; the free energy change per monomer becomes more negative with increasing temperature, indicating that the attraction between the monomers in a polymer is stronger at higher temperature. This effect of temperature is in contrast to that of ionic strength which affects mainly the equilibrium. The enthalpy change, entropy change, and heat capacity change for the association were also estimated and were indicative of the formation of a considerable amount of hydrophobic bonds upon association.

Application of the theory of gelation to enzymatic clotting process of casein micelle solution

Dynamic mechanical measurements were carried out to clarify the mechanism of the clotting process of casein micelle solution. It was found that the clotting process of casein micelle solution was formally expressed by a first order reaction. The enzyme concentration dependence of the latent time tL and the rate constant of gelation Kg were found to be tL alpha [E]-1.1, and Kg alpha [E]1.0, respectively. These results were intrepreted on the basis of the theory of gelation. The results obtained here were found to agree with the theoretical conjectures. The casein micelle concentration dependence of the complex rigidity was also studied.

Structure of Bovine αs-Casein

The secondary structure of bovine αs-casein and chemically modified αs-casein in various solvents was investigated by infrared absorption spectrum and optical rotatory dispersion measurements. Amino groups of αs-casein were either succinylated or acetylated, and carboxyl groups were either methylated or ethylated. Acetylated- and ethylated-αs-caseins are insoluble in water. Water-soluble samples have unordered structure in water. In organic solvents, such as 2-chloroethanol and ethylene glycol, they have about 50% α-helical fraction. On the other hand, it was found that methylated-αs-casein had two infrared absorption peaks centered at 1625 and 1643 cm−1 in D2O-CH3OD mixed solvent. This fact may be connected with the presence of β-structure. In the case of solid film of this sample, cast from solution containing CH3OH, the presence of β-structure was indicated, too. The authors attempted to explain the formation of β-structure in methylated-αs-casein in terms of the electrostatic interactions due to the d...

Influences of Temperature and Urea on Casein Micelle Components

温度や尿素によるカゼインミセル成分の変化を超遠心分離やゲル電気泳動で追究した.カゼインミセル懸濁液を4°Cおよび25°Cでそれぞれ保存すると22,000g80分の遠心分離で非沈降性のカゼイン,特にβ-カゼインが1～2時間以内に4°Cの場合は急激に増加し,25°Cの場合は減少し,そしてミセルのカぜインと上澄液中のカゼインとの間の平衡が4°Cでは4時間後に,25°Cでは2～3時間後にできた.次に4,8,12,20,25,30°Cの各温度で4時間保存した時の非沈降性β-カゼインの結果から見て冷却および加温の際のβ-カゼインの温度依存性をもつ解離,会合は主として12～20°Cの範囲で生ずることが考察された.尿素のミセル成分への影響は低温の場合と同様で1M尿素存在下では25°Cにおいても非沈降性カゼインは著しく増加した.尿素が1.5M存在するとβ-カぜインを多く含むおよそ5～10万の分子量をもつα-β-γカぜイン複合物がセファデックス(Sephadex)G200のゲル濾過でミセルから溶出した.これらの結果からβ-カぜインの多くは溶媒と接し易いところにあり,疎水結合を通してミセルと関与し.ミセル形成に重要な役割をもつことが考えられた.

A Study of Casein Micelle:I. Effect of Heat on Casein Micelle

1. カゼイン•ミセルにCa++を添加すると,ミセルが会合して白濁度が増加する.そこで,その白濁度を分光光度計で測定し,その値をカゼイン•ミセルのCa感度とした.吸光度の測定にあたつてpH7.0,0.02M硼酸緩衝液を含む0.1～0.2%カゼイン溶液に,CaCl2を加えて0～100mMとし,35°Cに保持して,10～16分の間にE1cm=610mμで測定した.カゼイン•ミセルとCa++との会合によつて生ずるミセルの増大化は,温度によつて大きく左右された.またミセルは,増大するにしたがつて膠質学的に不安定になり,沈殿性が増加した.2. カゼイン•ミセルをそれぞれ60,75,90°Cで10分間加熱処理し,Ca++を添加したところ,生じた白濁度の変化には加熱しないカゼイン•ミセルの場合と比較して,明らかに差が認められた.そこでカゼイン•ミセルは,加熱によつて変化することがわかつた.なお,pH7.0,0.1M硼酸緩衝液を,加熱処理の前に加える場合と,後に加える場合とでは,Ca感度に差があることが認められた.