Top right panel: continuum-subtracted Hα image of UGC 4286, the host galaxy of SN 2010al. The 25th B-band magnitude elliptic contour is shown with a black solid line, along with the position of SN 2010al (marked by a red star) and the center of the galaxy (marked by a red circle). The slit position is shown, and a color code is used to present the N2 metallicity measurements at the position of each H ii region that we inspected. Top left panel: flux at the Hα wavelength along the slit, shown as a function of the distance from the SN location (marked by a dotted line, like the nucleus position). The N2 measurements are shown at the corresponding positions in the top subpanel. Bottom panel: metallicity gradient of UGC 4286. The linear fit on our measurements is shown with a solid line. The interpolated metallicity at the SN distance is marked with a red square and its uncertainty corresponds to the fit error. The error bar (±0.2) for our N2 measurements is shown aside. The positions of SN and nucleus are marked with vertical dotted lines. The solar metallicity (Asplund et al. 2009) and the LMC metallicity (Russell & Dopita 1990) are indicated with two horizontal dotted lines.