SLC4A family members 4, 5, 7, and 9 can each couple the transport of bicarbonate (HCO3-) with sodium ions (Na+). SLC4A4 (aka NBCE1) is an electrogenic sodium/bicarbonate cotransporter with a Na+:HCO3- stoichiometry of 1:3, although it can also be 1:2 (Burnham et al. 1997). SLC4A4 encodes a protein which is expressed in the kidney and pancreas, with lesser expression in many other tissues (Abuladze et al. 1998). Mutations in SLC4A4 cause permanent isolated proximal renal tubular acidosis (pRTA) (results in accumulation of acid in the body due to a failure of the kidneys to effectively acidify urine) with ocular abnormalities (Igarashi et al. 1999).
Adenosylhomocysteinase 2 (AHCYL2) can upregulate ion-transporting proteins such as the electrogenic sodium bicarbonate cotransporter 1 (SLC4A4, aka NBCE1) (Yamaguchi & Ishikawa 2014).