Figure 3. Visualization of dosed EGCG and its phase II metabolites in tissue micro-regions.

(a–j) Simultaneous visualization of EGCG and its phase II metabolites in (a–e) liver and (f–j) kidney sections. Three different images are shown in (a, f) H&E staining, (b, g) optical microscopy, and MALDI-TOF-MS: (c, h) EGCG (m/z 457), (d, i) EGCG-sulfate (m/z 537), and (e, j) EGCG-glucuronide (m/z 633). An additional EGCG spot (red circle) was visualized as the positive and internal control. (k–m) For mass spectral identification, the isotopic fine structures were measured by MALDI-FT-ICR-MS in the region of interest (I, III, and V) indicated in panels h–j. Theoretical peaks of (k) EGCG (C₂₂H₁₇O₁₁), (l) EGCG-sulfate (C₂₂H₁₇O₁₄S₁), and (m) EGCG-glucuronide (C₂₈H₂₅O₁₇) were shown in negative ionization mode, and isotopic peaks were observed in the (M–H⁺ + 1)⁻ and (M–H⁺ + 2)⁻ regions. These peaks were theoretically assigned to the substitution of a stable isotope for each element. Asterisk shows background peaks.