头孢噻呋进入动物体内后会在短时间内被吸收,并代谢成同样具有抗菌活性的去呋喃甲酰基头孢噻呋(DFC)。根据我国农业部235号文件及欧盟的规定,在对头孢噻呋进行定量分析时应充分考虑去呋喃甲酰基头孢噻呋的含量。目前,用于检测头孢噻呋及其代谢物残留的主要方法有高效液相色谱(HPLC)法和高效液相色谱-串联质谱(LC-MS/MS)法。尽管HPLC法灵敏度和定性准确性相对较低,但已逐渐被质谱分析方法取代。然而,在复杂动物源性食品基质中使用低分辨质谱时,杂质峰的干扰会影响头孢噻呋的定量与定性准确性的提升。
Cefotaximase-Munich (CTX-M) extended-spectrum beta-lactamases (ESBLs) are commonly associated with Gram-negative, hospital-acquired infections worldwide. Several beta-lactamase inhibitors, such as clavulanate, are used to inhibit the activity of these enzymes. To understand the mechanism of CTX-M-15 activity, we have determined the crystal structures of CTX-M-15 in complex with two specific classes of beta-lactam compounds, desfuroylceftiofur (DFC) and ampicillin, and an inhibitor, clavulanic acid. The crystal structures revealed that Ser70 and five other residues (Lys73, Tyr105, Glu166, Ser130, and Ser237) participate in catalysis and binding of those compounds. Based on analysis of steady-state kinetics, thermodynamic data, and molecular docking to both wild-type and S70A mutant structures, we determined that CTX-M-15 has a similar affinity for all beta-lactam compounds (ceftiofur, nitrocefin, DFC, and ampicillin), but with lower affinity for clavulanic acid. A catalytic mechanism for tested β-lactams and two-step inhibition mechanism of clavulanic acid were proposed. CTX-M-15 showed a higher activity toward DFC and nitrocefin, but significantly lower activity toward ampicillin and ceftiofur. The interaction between CTX-M-15 and both ampicillin and ceftiofur displayed a higher entropic but lower enthalpic effect, compared with DFC and nitrocefin. DFC, a metabolite of ceftiofur, displayed lower entropy and higher enthalpy than ceftiofur. This finding suggests that compounds containing amine moiety (e.g., ampicillin) and the furfural moiety (e.g., ceftiofur) could hinder the hydrolytic activity of CTX-M-15.