The highest activity of the cytoplasmic enzyme ADA is found in lymphocytes in humans. When there is a deficiency of ADA, adenosine accumulates and is converted into its ribonucleotide or deoxyribonucleotide forms by cellular kinases. This leads to an increase in dATP levels, which inhibits ribonucleotide reductase and prevents the production of deoxyribose-containing nucleotides. As a result, cells are unable to make DNA and divide. This deficiency can cause a severe combined immunodeficiency disease (SCID), which is characterized by a decrease in T cells, B cells, and natural killer cells. Approximately 14% of SCID cases in the United States are attributed to ADA deficiency. Treatments for this disorder include bone marrow transplantation, enzyme replacement therapy, and gene therapy. Without appropriate treatment, children with ADA deficiency typically do not survive past the age of 2 years. Another form of immunodeficiency primarily affecting T cells is caused by purine nucleoside phosphorylase deficiency.

Unlike the synthesis of the purine ring, which occurs on a pre-existing ribose 5-phosphate, the pyrimidine ring is synthesized prior to being attached to ribose 5-phosphate. The atoms required for pyrimidine ring synthesis are derived from glutamine, CO2, and aspartate. The regulated step in this pathway is the synthesis of carbamoyl phosphate from glutamine and CO2, which is catalyzed by carbamoyl phosphate synthetase (CPS) II. CPS II is inhibited by uridine triphosphate, the end product of the pathway, but can be activated by PRPP. Carbamoyl phosphate is also a precursor of urea and defects in ornithine transcarbamylase of the urea cycle can promote pyrimidine synthesis. The formation of carbamoylaspartate, catalyzed by aspartate transcarbamoylase, is the second step in pyrimidine synthesis. The pyrimidine ring is then closed by dihydroorotase. The resulting dihydroorotate is oxidized to produce orotic acid (orotate). The enzyme responsible for orotate production, dihydroorotate dehydrogenase, is located in the inner mitochondrial membrane, while all other enzymes involved in pyrimidine biosynthesis are cytosolic. The first three enzymatic activities in this pathway (CPS II, aspartate transcarbamoylase, and dihydroorotase) are different domains of a single polypeptide known as CAD. Pyrimidine nucleotide synthesis is an essential process facilitated by multifunctional or multicatalytic polypeptides. The synthesis of purine nucleotide IMP also involves multifunctional proteins.
ADA is expressed in a variety of tissues, but, in humans, lymphocytes have the highest activity of this cytoplasmic enzyme. A deficiency of ADA results in an accumulation of adenosine, which is converted to its ribonucleotide or deoxyribonucleotide forms by cellular kinases. As dATP levels rise, ribonucleotide reductase is inhibited, thereby preventing the production of all deoxyribose-containing nucleotides (see p. 297). Consequently, cells cannot make DNA and divide. [Note: The dATP and adenosine that accumulate in ADA deficiency lead to developmental arrest and apoptosis of lymphocytes.] In its most severe form, this autosomal-recessive disorder causes a type of severe combined immunodeficiency disease (SCID), involving a decrease in T cells, B cells, and natural killer cells. ADA deficiency accounts for ~14% of cases of SCID in the United States. Treatments include bone marrow transplantation, enzyme replacement therapy, and gene therapy (see p. 501). Without appropriate treatment, children with this disorder usually die from infection by age 2 years. [Note: Purine nucleoside phosphorylase deficiency results in a less severe immunodeficiency primarily involving T cells.]
VI. PYRIMIDINE SYNTHESIS AND DEGRADATION
Unlike the synthesis of the purine ring, which is constructed on a pre-existing ribose 5-phosphate, the pyrimidine ring is synthesized before being attached to ribose 5-phosphate, which is donated by PRPP. The sources of the atoms in the pyrimidine ring are glutamine, CO2, and aspartate (Fig. 22.19).
A. Carbamoyl phosphate synthesis
The regulated step of this pathway in mammalian cells is the synthesis of carbamoyl phosphate from glutamine and CO2, catalyzed by carbamoyl phosphate synthetase (CPS) II. CPS II is inhibited by uridine triphosphate (the end product of this pathway, which can be converted into the other pyrimidine nucleotides) and is activated by PRPP. [Note: Carbamoyl phosphate, synthesized by CPS I, is also a precursor of urea (see p. 253). Defects in ornithine transcarbamylase of the urea cycle promote pyrimidine synthesis because of increased availability of carbamoyl phosphate. A comparison of the two enzymes is presented in Figure 22.20.]
B. Orotic acid synthesis
The second step in pyrimidine synthesis is the formation of carbamoylaspartate, catalyzed by aspartate transcarbamoylase. The pyrimidine ring is then closed by dihydroorotase. The resulting dihydroorotate is oxidized to produce orotic acid (orotate), as shown in Figure 22.21. The human enzyme that produces orotate, dihydroorotate dehydrogenase, is a flavin mononucleotide-containing protein of the inner mitochondrial membrane. All other enzymes in pyrimidine biosynthesis are cytosolic. [Note: The first three enzymic activities in this pathway (CPS II, aspartate transcarbamoylase, and dihydroorotase) are actually three different catalytic domains of a single polypeptide known as CAD from the first letter in the name of each domain. (See p. 18 for a discussion of domains.) This is an example of a multifunctional or multicatalytic polypeptide that facilitates the ordered synthesis of an important compound. Synthesis of the purine nucleotide IMP also involves multifunctional proteins.]
C. Pyrimidine nucleotide synthesis