Abstract
Summary
1) Bioassays of various pituitary preparations were made to compare quantitative and qualitative characteristics of the different MSH active principles known to be present in such preparations. Results suggest that the 3 chemically distinct pituitary fractions possessing MSH activity (i.e., alpha-MSH, beta-MSH, and ACTH) act on melanophores in a qualitatively similar manner. It is estimated that the intrinsic MSH activity of ACTH can not be greater than 1/10th that of alpha-MSH. 2) A melanophorotropic activity unit (MAU) is proposed which is defined as the amount of active principle producing 1/2 maximal response (or greater) in 50% of the treated animals. The approximate relationship of MAU to “units” used by other investigators is discussed. The frog bio-assay is sensitive to 0.01 MAU and therefore could be used to measure as little as 10-3 μg of pure ACTH (corresponding to 10-4 I.U. of ACTH) provided this fraction can be completely separated from the other MSH active principles by previous chemical treatments, and assuming that ACTH has an intrinsic MSH activity as shown recently.
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