To examine the intracellular localization of the pancreatic lipase in the mouse pancreas, both histochemical and immunohistochemical stainings were applied on the ultra-thin frozen sections. Furthermore, in order to study the mutual relationship of the distribution between lipase and α-amylase as well as α-amylase and chymotrypsinogen A within a zymogen granule, immunohistochemical double staining was performed.
The positive reaction for lipase by the histochemical staining were observed in the peripheral zone of the zymogen granules, as belt-like electron opaque deposits of 20Å to 40Å in width. Especially, in the isolated zymogen granules, the positive reaction sites were clearly recognized inside the limiting membrane of the zymogen granule as electron dense deposits of 30Å to 40Å in diameter. By the immunohistochemical study, lipase was localized along the limiting membrane of the zymogen granule, while, α-amylase and chymotrypsinogen A were distributed diffusely in a zymogen granule. Moreover, when the isolated zymogen granules were treated with alkaline solution, large amounts of lipase remained in the membrane fraction, but most α-amylase activity (more than 95%) was extracted into the soluble fraction. This fact suggests that lipase is more closely associated with the limiting membrane of the zymogen granule, than α-amylase.
From these results, the localizing pattern of the pancreatic lipase was found to be different from those of α-amylase, chymotrypsinogen A and trypsinogen; the former bound loosely to the inside of the limiting membrane of the zymogen granule, while the latter enzymes were localized diffusely within a zymogen granule.
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