The kynurenine pathway as a drug target

Hayaishi and tryptophan metabolism Osamu Hayaishi is the founder of biochemistry in Japan. After the 2 nd World War, he returned to Osaka University to take a position. Everything had been destroyed, so he had nothing to do research with. Professor Kotake gave him a bottle of tryptophan to do research. He isolated a bacterium from the soil outside the lab that could grow on tryptophan.

Hayaishi and tryptophan metabolism Hayaishi found that the bacterium, Pseudomonas fluorescens, used a pathway called the kynurenine pathway previously found in animals. This pathway was found in rats to be related to a nutrional deficiency called pelagra. The end products of the kynurenine pathway are different in animals and bacteria. Bacteria completely metabolize tryptophan to CO 2 and NH 3. Animals metabolize tryptophan to NAD(P).

The kynurenine pathway in Pseudomonas fluorescens

The kynurenine pathway in humans NH 3 + O N H O O 2 NH 3 + O NH O O OHC NH 3 + O NH 2 O O O 2, NADPH NH 3 + O NH 2 O O OH NH 3 + O O NH 2 O OH O O 2 N CO 2 - CO 2 - NAD(P)+ NH 2 O OH O O O CO 2 + H 2 O + NH 3 N CO liver niacin

Where is the kynurenine pathway found in the body? Most tryptophan metabolism occurs in the liver. However, Hayaishi found that it can occur in other tissues, such as lung, intestines, and placenta. The first enzyme in the pathway is different in liver (TDO) and the other tissues (IDO). Both enzymes contain a heme group like hemoglobin, so they bind O 2 and are red in color.

IDO and the fetal-maternal paradox IDO is found in the placenta, in cells contacting the maternal blood. The placenta is made of fetal cells, which are not the same as the mother. Why doesn't the mother's immune system attack the fetus? Andrew Mellor and David Munn at MCG showed that inhibition of IDO causes abortion in gravid mice. Thus, IDO regulates the immune response to determine self and other.

IDO and cancer IDO is also found in cancer cells. Although cancer cells are different than normal cells, they are not attacked by the immune system. An IDO inhibitor, 1-methyltryptophan, is now being investigated at MCG for anticancer activity The D-isomer seems to be more active in vivo.

How do IDO inhibitors affect the immune response? Possibly the depletion of tryptophan prevents T- cell activation. Other metabolites (kynurenines) may inhibit T- cell proliferation. Maybe both effects are involved.

Quinolinate and the central nervous system Another metabolite of the kynurenine pathway is quinolinate. Quinolinate is an excitatory neurotoxin due to binding at the NMDA receptor. Glutamate is the natural ligand for the NMDA receptor. Too much MSG can cause “Chinese restaurant syndrome” in some people.

Quinolinate and the central nervous system The kynurenine pathway is elevated by inflammation and infection. Increased levels of quinolinate are found in serum of patients with HIV-related dementia. Increased quinolinate has also been found in patients with Huntington's disease and Alzheimer's disease. Compounds which block quinolinate formation could be used to treat these diseases.

Kynurenine pathway enzymes are drug targets Kynurenines and quinolinate are neurotoxic. Reduction of these metabolites may be therapeutic for neurodegenerative diseases. Inhibition of kynurenine monoxygenase and kynureninase will reduce kynurenines and quinolinate.

The kynureninase family tree

Kynureninase sequences The amino acid sequence of the protein from bacteria to humans is only about 28% identical. Chimpanzees and humans differ in only 1 amino acid out of 465, so we are 99.6% identical. The human sequence is similar to dog (88.8%), rat (86.3%), mice (84.8%), chickens (72.5%), zebrafish (72.3%), and worms (51.5%), then fungi. The similarity is related to the evolutionary distance between organisms. These data provide a time-clock for evolution. Other proteins give similar results.

Alleles of kynureninase Even individual humans show variation in protien sequences. Several alleles of kynureninase have been found in humans. K412E and R188Q have been found in Han Chinese, and are linked with hypertension. T198A was found in a boy in Somalia, and he had high levels of excretion of kynurenic and xanthurenic acids.

The structure of human kynureninase

Comparison of bacterial and human kynureninase