Mechanisms of Intestinal Putrefaction
Not all dietary amino acids are absorbed in the small intestine. A fraction reaches the large intestine, serving as a nutrient source for resident microflora. Bacterial enzymes trigger decarboxylation and deamination reactions, converting useful molecules into harmful metabolites.
This breakdown yields a spectrum of compounds: biogenic amines, hydrogen sulfide, and cyclic structures including phenols, indole, and skatole. These substances are highly cytotoxic. They are partially absorbed in the lower GI tract, enter the systemic circulation via the portal vein, and are transported to the liver for emergency detoxification. The neutralized forms are subsequently excreted by the kidneys.
Tyrosine Derivatives: Cresol and Phenol
Under the influence of microbial enzymes, the amino acid tyrosine loses its side chains. This stepwise process leads to the formation of p-cresol followed by phenol.
Upon reaching the liver, these toxins are neutralized via conjugation reactions (coupling with polar molecules). There are two main pathways:
- Sulfation. Mediated by sulfotransferases. The active form of sulfuric acid, PAPS (3'-phosphoadenosine-5'-phosphosulfate), serves as the sulfate group donor. This yields cresolsulfate and phenolsulfate.
- Glucuronidation. Catalyzed by UDP-glucuronosyltransferase using UDP-glucuronate. The resulting products are phenol glucuronide and cresol glucuronide.
These conjugates are highly water-soluble and readily excreted in the urine. A sharp spike in the urinary concentration of these conjugates indicates heightened intestinal putrefaction.
Tryptophan Catabolism: Indole and Skatole
Another bacterial target is tryptophan. Microorganisms cleave the $\alpha$-amino and carboxyl groups while preserving the bulky heterocyclic ring.
Trypotophan degradation branches into two pathways:
- Direct cleavage of the side chain yields indole (with serine or alanine as byproducts).
- Synthesis of skatole proceeds via an indoleacetic acid intermediate (with release of carbon dioxide).
Two-Step Detoxification of Indole
Skatole and indole are strongly hydrophobic molecules. Because the liver cannot directly attach a polar group to them, detoxification proceeds strictly in two steps.
Step 1. Microsomal Oxidation To prepare a site for conjugation, the cytochrome P450 enzyme system hydroxylates the molecule. Indole interacts with oxygen in the presence of the NADPH cofactor, producing indoxyl.
Step 2. Conjugation (Sulfation) The resulting indoxyl is acted upon by sulfotransferase, utilizing PAPS as the donor to form indoxyl sulfate.
Its potassium (or sodium) salt is known as animal indican. Measuring the concentration of animal indican in urine is an important clinical tool for assessing hepatic detoxification capacity. The oxidation and conjugation of skatole follow an entirely analogous pathway (yielding skatoxyl followed by sulfate or glucuronic acid attachment).