Mammalian kidney and liver are critical in maintaining physiological ionic environment. Kidney specializes in removing toxins, drugs, and other organic anions from the blood by a process called "renal secretion". Besides kidney, anionic substrates are also transported in other organs, e.g., choroid plexus, eye, airway, and placenta. Brain possesses two physiological barriers, namely blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) which restrict entry of various compounds. Therefore, specialized proteins or transporters must reside at the basolateral and apical membranes that can transport various organic solutes against the gradient (active transport). The major, known renal secretory transport systems are multispecific and have been placed into two distinct groups: the organic anion transporter (OAT) and organic cation transporter (OCT). Several multispecific OATs (OAT1-3, OAT-K1 and OATK2) and OATPs (organic anion transporting polypeptides; oatp1-3), have been cloned and characterized from various tissues. OATPs family of proteins display high degree of sequence homology, and a general secondary protein structure (up to 12 transmembrane domains with cytoplasmic N and C-terminus).

Rat Oatp, now called oatp1, encodes a protein of 670 aa. Oatp1 was localized at the apical portions of the S3 segment of the proximal tubule, and at the apical surface of the choroid plexus of rat brain. It is also found in liver, lung, skeletal muscle, and colon. It mediates Na+-independent uptake of wide range of amphipathic substrates including sulfobromopthalein (BSP), bile acids, estrogen conjugates, and neutral steroids, etc.. Human liver OATP (670 aa) is only 67% homologous with rat oatp1. It differs in functional properties from rat oatp1 and highly expressed in brain.

Oatp2 (also called OATP-B1; 661 aa; 12 TM), cloned from rat brain, shares significant homology with rat oatp1 and OAT-K1 (77%) and human OATP (73%). It is highly expressed in brain, liver, and kidney but not in heart, spleen, lung, muscle, and testes. Oatp2 mediates uptake of taurocholate, estrogen conjugates, and cardiac glycosides ouabain. Unlike other oatps, oatp2 also transported digioxin.

Oatp3 (670 aa; 12 TM), cloned from rat retina, shares ~80-83% identity with oatp1-2 and ~77% to rat OAT-K1. Oatp3 is highly expressed in the kidney, and moderately in the retina. Oatp3 mediates uptake of taurocholate, thyroxine, and triiodothyronine. Most of the T4 secreted from the thyroid is deiodinated in peripheral tissues. The liver and kidney are the major sites for the production of T3 and T4. Therefore, oatp3 may play a critical role in transporting thyroid hormones from the circulation to the deiodination sites in the kidney.

Prostaglandins and thromboxanes play important physiological, pathological, and therapeutical roles in health and disease. PGT (prostaglandin transporter also known as martin F/G), a member of oatp family, mediates transport of PGs (PGD2, PGE1, PGE2, PGF2a) from the circulation. PGT (rat/human 643 aa; 12 TM) is expressed in brain, stomach, ileum, jejunum, and kidney but not in heart or skeletal muscle.

Most recently, a novel liver-specific OAT, LST-1, has been cloned from human liver. It has also been detected in mouse and rat liver. No significant expression of LST-1 was detected in other tissues. Human LST-1 (691 aa, 12 TM) shares 42% homology with human OATP, rat oatp1-3, and OATK-1, and 35% with PGT. Phylogenetic analyses places LST-1 between oatp and PGT. LST-1 has wide substrate specificity (taurocholate, conjugated steroids, PGE2, Thromboxane B2, leukotrienes, and T3). Therefore, LST-1 may be more important for clearance of bile acids and organic anions in liver.

ADI has produced highly specific rabbit antibodies for human OATP, oatp1-3, PGT, and LST-1 using antigenic peptide sequences unique to each protein. These antibodies do not crossreact with each other and can be used to study various transporters. Respective antigenic or control peptides are also available to confirm specificity of antibodies.

m=mouse; r=rat; h=human; ch=chicken; f=frog; ~CT or ~NT=near C or N-terminus. EC=Extracellular; CP=Cytoplasmic domain;

"Neat Antisera" are the unpurified antiserum and it is suitable for ELISA and Western.
"Affinity pure" antibodies have been over the antigen-affinity column and recommended for immunohistochemical applications.
"Control peptides" can not be used for Western as they are very short peptides. They are intended for ELISA or antibody competition studies.