AbstractThe enzymes of tetrahydrobiopterin synthesis have been studied in murine bone marow, in spleen, in erythrocytes, and in reticulocytes. Mice with chemically induced and with genetically conditioned reticulocytosis as found in the lactate dehydrogenase deficient strain (Ldh‐1c/Ldh‐1c) were used for analysis of reticulocytic enzyme activities. The activity of the biopterin synthesizing system is highest in bone marrow even though it amounts to only about 10 as compared with liver. The first enzyme of the biosynthetic pathway, GTP‐cyclohydrolase, virtually disappears during the final maturation step of reticulocytes. In contrast, the activities of 6‐pyruvoyltetrahydropterin synthase and of sepiapterin reductase of erythrocytes are only reduced to about one half of the reticulocyte level. The absence of biopterin in erythrocytes is therefore caused by the loss of the enzyme that initiates the pterin biosynthetic
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