preferred_name: xanthine(in) + H(+)(in) => xanthine(out) + H(+)(out)
preferred_name_lc: xanthine(in) + h(+)(in) => xanthine(out) + h(+)(out)
label: xanthine(in) + H(+)(in) => xanthine(out) + H(+)(out)
label_lc: xanthine(in) + h(+)(in) => xanthine(out) + h(+)(out)