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Two acid phosphatases have been demonstrated histochemically in mouse ventral prostate, seminal vesicles, coagulating glands, and liver and in human prostate. The first is the lysosomal acid phosphatase demonstrable by the Gomori technique. The second differs from thisβ-glycerophosphatase in that it splits naphthol AS phosphates but notβ-glycerophosphate; it has a different histochemical pH optimum and it is not inhibited by MoO4 or NaF. The enzyme does not represent the “tail” of alkaline phosphatase activity as it is not inhibited by inhibitors of alkaline phosphatase and it has a different localization in liver and in human prostate. The enzyme may be membrane-bound but a lysosomal localization has still to be confirmed.

Starved and fed mice were injected intravenously with either fluorescent or125I-insulin and the localization of the hormone was investigated in various tissues. The fluorescent hormone was found in the kidney of starved animals only, whereas the radioactive hormone was found in the proximal convoluted tubule cells of the kidney of both fed and fasted animals, although the latter group appeared to contain more radioactivity than the first. With high resolution techniques the radioactivity in the kidney was found in the brush border, the apical vacuoles, the apical mitochondria and the nucleus, and never in the Golgi apparatus or the lysosomes. All other tissues contained radioactivity due to the hormone, but no difference in either the amount or the localization was found between fed an starved animals. It is concluded that the process of absorption of insulin by the cells of the proximal convoluted tubules is specific and that the hormone is not degraded in a way similar to other proteins, but is likely to be stored and to control cellular metabolic processes from its sites of localization.