A modified Hummel-Dreyer equilibrium chromatography technique was used to test the hypothesis that NADH induces the molecular association of lactate dehydrogenase (LDH) and alpha-glycerol-3-phosphate dehydrogenase (alpha-GDH). In the presence of a very limited NADH concentration, a unique elution profile with a new peak running immediately ahead of a trough at the free alpha-GDH elution position is obtained. The appearance of this peak-trough profile is physical evidence that reversible association between LDH and alpha-GDH occurs over a very limited range of free NADH concentrations. The association constant for this complex formation between LDH and alpha-GDH is estimated to be 2.0 microM-1. With the NADH concentration increased to saturation level, no evidence of binding is observed. Such concentration-dependent behavior suggests that the strong competition between LDH and alpha-GDH for the limited amount of NADH tends to promote the enzyme-enzyme contact in order to make the most efficient use of the shared metabolite. The experimental results described in this article make a convincing argument for a metabolite-modulated enzyme-enzyme interaction along this metabolic pathway.

It has very recently been reported that deoxyribonucleic acid oligomers of cytosine with sequences such as d‐T(CN)T aggregate into tetrastranded sructures [J. L. Leroy et al. (1993), Biochemistry, Vol. 32, pp. 6019–6031; K. Gehring et al. (1993), Nature, Vol. 363, pp. 561–565; S. Ahmed (1994), Structural Biology, Vol. 1, pp. 83–88]. Using gel filtration chromatography we have found that the oligomer dC10 aggregates into a mixture of duplex, tetraplex, and octaplex structures. We have also found that at the concentration used for Raman spectroscopy (0.05 M in base residues), these structures remain stable from pH 5 to pH 8 at 5°C. The Raman spectra of these oligomers in a 0.1 M NaCl solution at pH 7 and 5°C show a remarkable resemblance to the Raman spectra of the A‐form double‐helical ribonucleic acid polymer of cytosine taken at pH 5.5 and room temperature [C. H. Chen and G. J. Thomas, Jr. (1977), Biopolymers, Vol. 16, pp. 765–789]. This appears to be the first time that this A‐type furanose ring pucker has been reported in deoxyoligonucleotides in aqueous solution at low salt and pH 5.5–7. The gel filtration chromatography and the uv melting behavior of the annealed dC10 solutions show the presence of an equilibrium mixture of multiplexes with multiple melting transitions. Very slow annealing of dC10 solutions in the pH range 6.5–7.0 also produced a similar equilibrium mixture of multiplexes, but at a much slower rate. Rapidly cooled samples tended to change to the equilibrium mixture over a period of several days. © 1997 John Wiley & Sons, Inc.