Beach J R, Henning S J
Department of Biology, University of Houston, Texas 77004.
Pediatr Res. 1988 Jan;23(1):58-62. doi: 10.1203/00006450-198801000-00013.
Milk can be a significant source of lead (Pb) for young mammals, including humans. Certain essential trace elements have previously been shown to be specifically associated with particular milk components and such associations often increase bioavailability. Thus, the first goal of this study was to determine the distribution of Pb in cream, casein, and whey fractions of various milks under various conditions using 203Pb as a tracer. In rat milk almost 90% of the Pb was found to be associated with the casein micelles, regardless of: 1) whether the milk was labeled in vivo or in vitro; b) whether the milk was fresh or frozen; and c) the added concentration of Pb (over the range 0.01-75 micrograms/ml). The remainder of the Pb was approximately equally distributed between cream and whey. A virtually identical pattern of Pb distribution was observed with bovine milk. Pb added to infant formula also associated predominantly with casein micelles, although the Pb content of this fraction was significantly less than with rat and bovine milks. The second goal of the study was to determine if Pb remained associated with casein as it traversed the gastrointestinal tract of infant rats. For this purpose, rat pups aged 15-16 days were gavaged with 203Pb-labeled rat milk, and lumenal contents from the stomach and small intestine were collected 2 h later. Differential centrifugation of the homogenized lumenal contents showed that in the stomach the Pb was associated primarily with the casein curd. By the time chyme reached the distal small intestine, Pb was found predominantly in a fraction that was not precipitable by high-speed centrifugation (thus, not intact casein micelles), but was nondialyzable. We conclude that Pb in milk is protein bound and remains this way as it traverses the stomach and proximal small intestine of the infant rat.
对于包括人类在内的幼年哺乳动物而言，牛奶可能是铅（Pb）的重要来源。此前已有研究表明，某些必需微量元素与特定的牛奶成分存在特定关联，且这种关联通常会提高生物利用率。因此，本研究的首要目标是，以203Pb作为示踪剂，测定在不同条件下各种牛奶的乳脂、酪蛋白和乳清组分中铅的分布情况。研究发现，在大鼠乳汁中，无论以下哪种情况：1）乳汁是在体内还是体外标记；b）乳汁是新鲜的还是冷冻的；c）添加的铅浓度（范围为0.01 - 75微克/毫升），近90%的铅都与酪蛋白胶粒相关联。其余的铅在乳脂和乳清中分布大致相等。在牛乳中也观察到了几乎相同的铅分布模式。添加到婴儿配方奶粉中的铅也主要与酪蛋白胶粒相关联，尽管该组分中的铅含量明显低于大鼠乳和牛乳。本研究的第二个目标是确定铅在幼鼠胃肠道中移动时是否仍与酪蛋白结合。为此，给15 - 16日龄的幼鼠灌胃203Pb标记的大鼠乳汁，并在2小时后收集胃和小肠的肠腔内容物。对匀浆后的肠腔内容物进行差速离心显示，在胃中，铅主要与酪蛋白凝块相关联。当食糜到达小肠远端时，发现铅主要存在于高速离心后不可沉淀（因此不是完整的酪蛋白胶粒）但不可透析的组分中。我们得出结论，牛奶中的铅与蛋白质结合，并在幼鼠的胃和小肠近端移动过程中一直保持这种状态。
