Parition of riboflavin in cow's milk

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Abstract
1. The partition of riboflavin between the free and the protein-bound forms was measured in the milk of six Ayrshire cows, three of which were treated with l-thyroxine. 2. As lactation progressed there was a slow decline in the total riboflavin content of the milk for which the initial values were 0·86 to 1·09 µg./ml. milk. The rate of decline ranged from 0·01 to 0·003 µg./ml. milk per day. 3. Thyroxine had its expected effects in stimulating milk yield and heart rate, and in preventing the rise of phosphatase in the milk as lactation progressed, but it did not affect either the concentration or the partition of riboflavin. 4. Reasons are advanced for the marked contrast between the effect of thyroxine in increasing the phosphorylation of thiamine and the absence of any such effect on riboflavin. We thank Miss S. McLauchlan and Miss M. Lightbody for technical assistance, and Glaxo Laboratories Ltd., Greenford, Middlesex, for a gift of L-thyroxine.