1. The histochemical dephosphorylation of riboflavin phosphate and pyridoxal phosphate has been studied at pH 9.0 and pH 5.0. The results have been compared with those obtained with glycerophosphate.

2. It was concluded that these substrates are dephosphorylated by enzymes or enzyme systems differing from each other and from that which dephosphorylates glycerophosphate.

3. The vitamin substrates undergo some dephosphorylation by all tissues examined at pH 9.0. At pH 5.0, however, no reaction is given by any tissue with riboflavin phosphate as a substrate, but all tissues give some reaction at this pH with pyridoxal phosphate.

4. The reaction at both acid and alkaline pHs with pyridoxal phosphate is in some tissues located in bodies resembling mitochondria: the sarcosomes of heart-muscle in particular give a strong reaction. This is true at pH 9.0, to a lesser extent, with riboflavin phosphate.

5. It is possible that the sites of dephosphorylation shown by the histochemical tests may also be the sites of phosphorylation of these vitamins.

6. Intercalated disks of heart-muscle show an intense dephosphorylating activity for riboflavin phosphate and glycerophosphate.

7. Pyridoxal phosphate and riboflavin phosphate are dephosphorylated at the same sites (differentiating regions) as glycerophosphate in rat embryos.

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