1. The enzyme involved in the synthesis of porphobilinogen from δ-aminolevulinic acid (ALA dehydrase) has been shown to be present in reduced levels of activity in the livers of mice bearing a number of transplanted tumors.

  2. From the data thus far available, the variations in ALA dehydrase activity levels resemble those previously found for catalase in that (a) pregnancy does not lower the hepatic activity of either enzyme and (b) the levels of both enzymes are much lower normally in the C57BL strain of mice than in others thus far examined and do not appear to be reduced further by tumors.

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