Screening of Inhibitory Activity of Narrow Fractions of Humic Substances towards Different Types of Class A β-Lactamases

Inhibition of β-lactamases involved in the development of bacterial resistance to β-lactam antibiotics is promising for restoring their effectiveness as antibacterial drugs. In this work, we screened the inhibitory activity against some serine β-lactamases of class A among humic substances of various origins, which represent complex organic matrices, as well as their narrow fractions obtained by separating preparations by acidity and polarity. The structural and group composition of broad fractions was characterized by 13C NMR spectroscopy; the molecular composition of narrow fractions was characterized by Fourier transform-ion cyclotron resonance mass spectrometry. The widest specificity of inhibition against various ESBL (TEM-12, TEM-17, TEM-18) and the inhibitor-resistant β-lactamase TEM-34 was possessed by the fraction of humic acids of brown coal CHA-I, isolated at pH 9: the values of residual activity of TEM -12, TEM-17, TEM-18, and TEM-34 were (%) 52 ± 5, 52 ± 5, 40 ± 4, and 58 ± 6, respectively. The data obtained for the most active narrow fraction turned out to be comparable to the effectiveness of sulfohydantoins, a promising new class of inhibitors. This indicates the feasibility of further work on the fine fractionation of natural humic substances using 2D chromatography, which will open a new, extensive source of β-lactamase inhibitors of non-β-lactam nature.