Silk glands of several spiders and caterpillars contain powerful cytoplasmic phosphatase activity localized along the inner border of the cells next to the gland lumen. Phosphatase activity is also high in the nuclei, particularly the nucleoli. Reservoir regions which secrete no silk lack the phosphatase border.

Although it is most easily demonstrated histochemically at pH 9, phosphatase activity in sections or homogenates of the glands is high over a considerable range of pH, from at least pH 5.5 to pH 9.8. It appears to be inhibited both by sulphydryl inactivators and by an abundance of sulphydryl groups. In view of the latter point, it is interesting to note that no sulphydryl groups can be detected in the phosphatase border zone (in the more insoluble constituents which remain informalin-fixed frozen sections), although the main bulk of th e cytoplasm is rich in sulphydryl groups.

Except in the non-secretory reservoir regions, silk glands of all the species examined are rich in cytoplasmic ribonucleic acid. Although the latter is abundant in the main body of the cell, it is, however, absent from the phosphatase border zone, so that in the cytoplasm nucleic acid ends where phosphatase begins.

This suggests that the phosphatas e may be par t of a system of enzymes involved in liberating the finished protein from acomplex with nucleic acids (which have been concerned either in the synthesis of the protein, or in protecting it from proteases, or both). This possibility is discussed in the light of previous studies of phosphatase-nucleic acid relations in other tissues.

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