Molecular & Biochemistry

1. Taurine: The Essential Amino Acid

Cats lack the hepatic activity of cysteine sulfinic acid decarboxylase (CSAD)and have low cysteamine dioxygenase — the two enzymes humans use to synthesise taurine from cysteine. The result: dietary taurine is essential. Taurine deficiency produces the classical Hayes & Sturman 1981 syndrome of central retinal degeneration (FCRD) and dilated cardiomyopathy — the latter eliminated from commercial pet food after a 1987 mandate to fortify cat food with 0.1 % taurine.

Mechanistically, taurine forms taurine-conjugated bile acids (taurocholate, taurochenodeoxycholate) — obligate in cats, optional in mice/humans — and modulates osmotic balance in retinal photoreceptors and cardiomyocytes via membrane-bound transporters and Ca²⁺ handling.

2. Vitamin A and the Inactive UGT1A6

Cats cannot convert β-carotene to retinol — the β-carotene 15,15′-dioxygenase (BCMO1) is enzymatically inactive. They must consume preformed vitamin A from animal sources. Combined with their inability to glucuronidate many xenobiotics (the UGT1A6 pseudogene, Court & Greenblatt 1997), this makes the cat acutely sensitive to plant alkaloids, paracetamol (acetaminophen), and many phenolic drugs.

The pseudogenisation of UGT1A6 across the entire Carnivora is consistent with millions of years on a hypocarnivorous diet where xenobiotic exposure was negligible. The relict molecular feature has clinical consequences: a single 500-mg paracetamol dose is lethal to a 4-kg cat (LD₅₀ ~50 mg/kg vs. ~150 mg/kg in humans).

3. Catnip and Silvervine: The 2-AP / Nepetalactone Receptors

The cat’s response to Nepeta cataria (catnip) is mediated by nepetalactone, an iridoid monoterpene. The molecular target was historically thought to be a vomeronasal receptor, but Uenoyama et al. (2021, Sci. Adv.) showed the active mechanism in Felis catus involves the μ-opioid receptor (OPRM1) system: nepetalactone exposure triggers endogenous β-endorphin release, with the characteristic head-rubbing and rolling response abolished by naloxone pretreatment.

The non-iridoid “catnip alternatives” — silvervine (Actinidia polygama), valerian, Tatarian honeysuckle — activate the same circuit through different ligands (nepetalactol, dihydroactinidiolide). About 30 % of domestic cats are non-responders, with a heritable component on chromosome E1 still being mapped.

4. The Tapetum Lucidum: Riboflavin Crystal Optics

The retinal “eye-shine” layer in cats is the tapetum lucidum cellulosum, a layer of ~15 cell-rows beneath the retinal pigment epithelium each containing parallel tetragonal crystallites of riboflavin (vitamin B2) and zinc (the riboflavin-zinc complex first identified by Pirie 1959). The crystallites are arranged with their \(c\)-axes perpendicular to the light-propagation direction, producing a quarter-wave dielectric mirror that maximally reflects in the green-to-yellow band — matching the peak sensitivity of feline rods.

The yellow-green eye-shine of F. catus reflects this optical tuning. By comparison, the dog tapetum uses guanine crystallites (also tetragonal but in different orientation), and the eye-shine peaks more in the blue.

5. Pheromones: Felinine, F3 & F4

Domestic-cat urine contains felinine (2-amino-7-hydroxy-5,5-dimethyl-4-thiaheptanoic acid), a sulphur-containing amino acid synthesised in the kidney from cysteine and isovaleryl-CoA via cauxin. Felinine is volatised by skin and bacterial flora to the characteristic pungent thiols of male-cat urine that mark territory. Cauxin, the limiting enzyme, is androgen-induced — explaining the much higher felinine output in intact toms.

The F3 facial pheromone (cheek-rub secretions, marketed as Feliway), a mixture of fatty acid esters, signals territorial security and is one of the few pheromone-based behavioural products with modest controlled-trial evidence for stress reduction. The F4 “allomarking” pheromone from chin glands signals familiarity to other cats.

6. Coat Pigmentation Biochemistry

Module 6 covers the biophysics of Turing-type colour patterning. The molecular biology underneath:

• MC1R / Agouti / TYR axis: eumelanin vs phaeomelanin choice in the melanocyte. Loss-of-function MC1R variants give pure phaeomelanin (red/orange/cream). The Orange (O) locus on the X-chromosome encodes a long non-coding RNA in a regulatory region affecting MC1R expression tissue-specifically — the basis of calico/tortoiseshell coat genetics in heterozygous females through random X-inactivation.
• TYR temperature-sensitive variantsproduce point mutations whose enzymatic activity drops sharply above ~33 °C. The Siamese point pattern — dark face, ears, paws, tail (cooler regions), pale body (warmer) — is the visible map of intracellular temperature.
• KIT & white-spotting: KIT pathway disruption blocks melanocyte migration from neural crest, producing the white belly/face/paws pattern (S-locus). Severe KIT lesions also cause sensorineural deafness when melanocytes fail to populate the cochlear stria vascularis.

7. Feline Blood Types & AB Antigen Biosynthesis

The feline AB blood-group system (Auer & Bell 1981) is encoded by CMAH (cytidine monophospho-N-acetylneuraminic acid hydroxylase). Functional CMAH produces N-glycolylneuraminic acid (Neu5Gc, blood group A); a frameshift variant produces N-acetylneuraminic acid (Neu5Ac, blood group B); the rare AB phenotype carries both. Naturally occurring strong anti-A antibodies in B-type cats make blood transfusion mismatches catastrophic — fatal within minutes of delivery. Pre-transfusion crossmatching is therefore mandatory, unlike in dogs where naturally-occurring isoantibodies are weak.

8. FIV Lentivirus & Species-Specific Restriction Factors

Feline immunodeficiency virus (FIV) is a lentivirus distinct from but closely related to HIV-1 and SIV. Its accessory protein Vif degrades the feline orthologue of the cytidine-deaminase restriction factor APOBEC3 to permit replication. The molecular biology of FIV-Vif–APOBEC3 interaction parallels HIV biology and has made FIV a key model for retroviral evolution. FIV is essentially non-pathogenic in wild felids despite high seroprevalence — a contrast with the catastrophic progression in domestic cats that the field is still working to explain at the molecular level.