1. Patients with pernicious anemia who are maintained on folic acid, 30 mg. three times a week, for two to three years may have a hematologic relapse which will remit satisfactorily if refined liver extract is given, or partially if the dose of folic acid is increased to 50 mg. daily, or if thymine is given.

2. The hematologic remission succeeding the increased dosage of folic acid is followed within several months by a second relapse. At this time the response of these patients to liver extract or vitamin B12 is retarded. Recovery occurs after two to four months.

3. These experiments suggest that folic acid exerts its effect by "mass action" in pernicious anemia and that it is essential to the formation of thymine and other pyrimidines and purines or facilitates the utilization of these substances.

4. Posterolateral column disease or peripheral neuritis occurred in every person with pernicious anemia who received increasing doses of folic acid to maintain an hematologic remission. This observation suggests that folic acid, by pushing a chemical reaction through to completion in the face of a serious deficiency of vitamin B12, depleted the supply of this factor even more and led to the development of combined system disease.

5. Uracil produced a hematologic response in 2 of 3 persons with pernicious anemia in relapse when given in doses of 15-30 grams daily. The data suggest that uracil may be a precursor of thymine.

6. A patient with pernicious anemia of pregnancy failed to respond to uracil, 30 grams daily, but did respond partially when choline, 3 grams, and methionine, 6 grams were given. Thymine induced a complete response. The partial response to methionine and choline and the better response to thymine suggest that choline and methionine supplied methyl groups for the formation of thymine, but that activating substances for the methylating process were missing.

7. Reference is made to a patient previously reported from this laboratory who had liver extract and vitamin B12-refractory megaloblastic anemia but who responded to folic acid and on a second relapse to thymine. Studies on the output of folic acid in the urine of this patient did not support the possibility of folic acid deficiency, and the suggestion was made that another substance, possibly the "Wills’ factor," was deficient, and that this factor together with folic acid activated the formation of thymine. These two factors correspond to the activators of the transmethylation reaction mentioned in the preceding paragraph.

8. These studies on human beings and similar ones in bacterial metabolism suggest that folic acid, liver extract and vitamin B12 are essential to the formation of nucleic acid and nucleoprotein through a chemical chain reaction. The suggestion is made that the megaloblast common to pernicious anemia and related macrocytic anemias is a primitive erythroblast with an abnormality in the metabolism of nucleoprotein. The so-called maturation arrest in all marrow elements occurs because of this abnormality which may be induced by a deficiency of vitamin B12, folic acid, the "Wills’ factor," and probably other chemical activators of this reaction.

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