The topographical distribution of GSSG reductase activity has been investigated at the histological level in solid unicentric transplants of three different types of autonomous mouse tumors. Assays have been performed on fresh frozen serial microtome sections, with an average protein content of about 20 µg. This technic permits sufficient separation in space for sampling of the peripheral tumor regions containing the young growing cells and the more central regions carrying older cell generations.

In continuous series of tumor sections there has been no proportionality between the observed activity and the pertinent tumor cell numbers per sample. The young and rapidly growing tumor cells have at least 4–6 times as much GSSG reductase activity per cell than the still vital nongrowing central tumor cell types.

The distribution of GSSG reductase runs closely parallel to the catheptic activity of the same tumors.

The GSSG reductase activity of solid tumors has been compared with that of ascites tumor cells and some normal mouse tissue. The calculated activity of the investigated growing tumor cell is about twice that of normal mouse liver cells. The average activity of two ascites tumor cell populations was found to be low, and that of cultured strain L cells was comparable with the activity of normal mouse fibroblasts.

The marked biochemical differences between the two main classes of cells in solid tumor transplants have been emphasized.

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This investigation was supported by institutional grants from the King Gustaf V's Jubilee Fund, the Swedish Cancer Society, and the Jane Coffin Childs Memorial Fund, all of which are gratefully acknowledged.

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