Affinity chromatography of the Neurospora NADP-specific glutamate dehydrogenase, its mutational variants and hybrid hexamers

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Abstract
The synthesis of an affinity adsorbent, 8-(6-aminohexyl)aminoadenosine 2''-phosphate-Sepharose 4B, is described. The assembly of the 2''-AMP ligand and the hexanediamine spacer arm was synthesized in free solution before its attachment to the Sepharose matrix. This adsorbent retarded the hexameric NADP-specific glutamate dehydrogenase [EC 1.4.1.4] of N. crassa, showing a capacity for this enzyme similar to that of comparable coenzyme-analog adsorbents for other dehydrogenases. The enzyme was eluted either at pH 6.8 in a concentration gradient of NADP+, or at pH 8.5 in the presence of NADP+ in concentration gradients of either dicarboxylates or NaCl. Anomalous effects of dicarboxylates in facilitating elution are discussed. 2''-AMP and its derivatives, 8-bromoadenosine 2''-phosphate and 8-(6-aminohexyl)aminoadenosine 2''-phosphate, which were used in the synthesis of the adsorbent, all acted as enzyme inhibitors competitive with NADP+. The chromatographic properties of the wild-type enzyme were compared with those of mutationally modified variants containing defined amino acid substitutions. This approach was used to assess the biospecificity of adsorption and elution and the contribution of non-specific binding. The adsorbent showed a low capacity for the enzyme from mutant am1 (Ser-336 replaced by Phe), a variant that has a localized defect in NADP binding, but an otherwise almost normal conformation, suggesting that non-specific interactions are at most weak. The enzyme from mutant am3, a variant modified in a conformation equilibrium, was fully retarded by the adsorbent, but showed a significantly earlier elution position than the wild-type enzyme. This is consistent with measurements in free solution that showed the am3 enzyme to have a higher Ki for 2''-AMP than the wild-type enzyme. The enzyme from mutant am19 was eluted as 2 distinct peaks at both pH 6.8 and 8.5. The adsorbent was used to separate hybrid hexamers constructed in vitro by a freeze-thaw procedure from pairs of purified variants. Several chromatographically distinct peaks of differing enzymological properties were purified from each hybridization mixture in quantities of up to a few mg, and represented distinct species of hybrid hexamers differing in subunit ratio.