Lee H S, Sen L C, Clifford A J, Whitaker J R, Feeney R E
J Nutr. 1978 Apr;108(4):687-97. doi: 10.1093/jn/108.4.687.
Casein epsilon-aminolysyl residues were converted to the methyl (and dimethyl), isopropyl or cyclopentyl derivatives in high yield with formaldehyde, acetone or cyclopentanone, respectively, in the presence of sodium borohydride. When incorporated into diets at 10% as the sole protein source, the chemically modified caseins failed to support growth of young rats. Methyl casein did, however, support limited growth after about 5 days. Plasma threonine levels increased and lysine levels decreased markedly in rats fed the alkyl caseins. The respective alkyllsine derivatives were present in plasma and urine. In another experiment, nearly normal or normal growth was obtained by feeding lysine-supplemented methyl or isopropyl casein, respectively. A preparation of partially methylated casein, containing approximately equal amounts of monomethyl- and dimethyllysines, supported normal rat growth. These results demonstrate that lysine deficiency was produced by feeding highly alkylated caseins. Digestibility of the chemically modified caseins in vivo was not affected, although in vitro studies with trypsin and alpha-chymotrypsin showed lowered digestibility. Since no apparent toxicity was observed limited methylation of food proteins may be useful for protection of lysyl residues against deteriorative reactions during processing and storage.
在硼氢化钠存在的情况下，分别用甲醛、丙酮或环戊酮将酪蛋白的ε-氨基赖氨酰残基高产率地转化为甲基（和二甲基）、异丙基或环戊基衍生物。当以10%的比例作为唯一蛋白质来源掺入日粮中时，化学修饰的酪蛋白无法支持幼鼠生长。然而，甲基酪蛋白在大约5天后确实支持有限的生长。喂食烷基酪蛋白的大鼠血浆苏氨酸水平升高，赖氨酸水平显著降低。相应的烷基赖氨酸衍生物存在于血浆和尿液中。在另一个实验中，分别喂食补充赖氨酸的甲基或异丙基酪蛋白，获得了接近正常或正常的生长。一种含有近似等量单甲基赖氨酸和二甲基赖氨酸的部分甲基化酪蛋白制剂支持大鼠正常生长。这些结果表明，喂食高度烷基化酪蛋白会导致赖氨酸缺乏。化学修饰酪蛋白在体内的消化率不受影响，尽管用胰蛋白酶和α-胰凝乳蛋白酶进行的体外研究表明消化率降低。由于未观察到明显毒性，食品蛋白质的有限甲基化可能有助于保护赖氨酰残基在加工和储存过程中免受降解反应的影响。
