The interaction of aflatoxin B1 with polynucleotides and its effect on ribonucleic acid polymerase

Abstract

1. The interaction of aflatoxin B₁ with different polynucleotides was studied spectrophotometrically. Equations were derived that enable the degree of binding to be determined without first determining the extinction coefficient of the bound form. 2. The interaction with calf thymus DNA obeys first-order relationships with an association constant of 0·40mm⁻¹, but there is some evidence for a secondary binding process from results obtained at 390nm. 3. The spectral shifts decreased in the order polyadenylic acid+polyuridylic acid>DNA>polyadenylic acid>polyadenylic acid+polyinosinic acid. Polycytidylic acid, polyuridylic acid, polyinosinic acid (both single- and triple-stranded), AMP, CMP, GMP and UMP did not interact with aflatoxin. It was concluded that there is a requirement for the amino group of adenine (or possibly guanine) for binding of aflatoxin to polynucleotides to occur. 4. Binding is reversed by increasing ionic strength, and by Mn²⁺ and Mg²⁺ in the concentration range studied (0–5mm). The effect of the Mn²⁺ or Mg²⁺ was far greater than would be expected on the basis of their ionic strength. With both the bivalent cations and sodium chloride the reversal is greatest with double-stranded polynucleotides. 5. Inhibition in vitro of the DNA-dependent RNA polymerase of Escherichia coli by aflatoxin B₁ was detected only in the absence of Mg²⁺ and at concentrations of Mn²⁺ below the optimum for RNA synthesis in vitro. 6. The degree of inhibition (maximally 30%) was dependent on the concentration of Mn²⁺ and decreased during incubation.