<p> Dihydrofolate reductase from a subline of the L1210 lymphoma was purified by affinity chromatography using substituted Sepharose -4B to which was coupled methotrexate, a specific, tight binding inhibitor of the enzyme. The purified enzyme was subjected to disc gel electrophoresis at pH 8.5. At least two bands of activity were detected on the gel by the formation of a reduced formazan. Their ratios were dependent on enzyme concentration. Similar bands were found in the presence of EDTA (10^-6M), 4M and SM urea. When a substrate, NADPH (5xl0^-5M), was added to the buffers used in electrophoresis, three bands of enzyme activity were present in a fixed ratio which was independent of enzyme concentration. Protein bands showed a different but constant ratio. When folate replaced dihydrofolate as substrate in the assay mixture, the bands of activity corresponded at high concentrations of the enzyme. When activity was detected in the presence of an increasing concentration of methotrexate, different inhibition of the bands resulted. Preliminary experiments with crude extracts of the same subline gave activity profiles with multiple peaks.</p> / Thesis / Master of Science (MSc)

Methodology was developed which allowed the rapid and routine quantitation of subpicomole quantities of diadenosine-5ʹ,5ʹʹʹ-P¹,P⁴-tetraphosphate (Ap₄A) in cultured mammalian cells. This methodology includes the rapid extraction of cellular nucleotides in cold alkali, resolution of Ap₄A from the bulk of cellular materials on a highly specific boronate affinity resin, and quantitation of the dinucleotide in a coupled bioluminescence assay utilizing venom phosphodiesterase and firefly luciferase. The sensitivity and selectivity of this assay is demonstrated and contrasted with previously developed techniques. This assay was used to examine the role of Ap₄A in DNA replication and the cellular stress response.