Total balance: intake = output

Positive balance: intake > excretion

Negative balance: intake＜output

Significance: Reflects the overview of protein synthesis and catabolism in the body

The nutritional value of protein depends on the type and proportion of essential amino acids

In the stomach: pepsin

Small intestine:

Definition: Undigested proteins and unabsorbed digestive products are broken down by intestinal bacteria in the lower colon.

Most of the products are harmful, such as amines, ammonia, phenol, indole, etc.

Hepatic encephalopathy—pseudoneurotransmitter theory

Two degradation pathways of proteins in eukaryotic cells

The amino acids absorbed by digestion and absorption of food proteins (exogenous) are mixed with the amino acids produced by the degradation of tissue proteins in the body and the non-essential amino acids synthesized in the body (endogenous)

Deamination: refers to the process of removing the α-amino group from amino acids to generate the corresponding α-keto acid.

Transamination: Under the action of transaminase (aminotransferase), a certain amino acid removes the α-amino group to form the corresponding α-keto acid, and another α-keto acid obtains this amino group to form the corresponding amino acid.

L-glutamate dehydrogenase catalyzes the oxidative deamination of L-glutamate to generate α-ketoglutarate and ammonia

Amino acids are deaminated through the purine nucleotide cycle

Alpha-keto acids can be completely oxidized and decomposed to release energy

Amination of alpha-keto acids produces nutritionally non-essential amino acids

Alpha-keto acids can be converted into sugars and lipids

Amino acid deamination and amine breakdown

Intestinal bacterial putrefaction

Renal tubular epithelial cell secretion

Ammonia is transported from skeletal muscle to the liver via the alanine-glucose cycle

Ammonia is transported from tissues such as the brain and skeletal muscles to the liver or kidneys via glutamine

Urea is synthesized via the ornithine cycle (urea cycle)

Regulation of urea synthesis

Urea synthesis disorder, elevated blood ammonia concentration

Decarboxylation of glutamic acid produces γ-aminobutyric acid (GABA, an inhibitory neurotransmitter, which has an inhibitory effect on the central nervous system)

Decarboxylation of histamine produces histamine (a strong vasodilator that can increase capillary permeability, smooth muscle contraction, and stimulate pepsinogen and gastric acid secretion)

Tryptophan is hydroxylated and then decarboxylated to generate 5-hydroxytryptamine (5-HT, which acts as an inhibitory neurotransmitter in the brain and constricts blood vessels in peripheral tissues)

Decarboxylation of certain amino acids can produce polyamines (regulate cell growth)

Tetrahydrofolate (FH4) participates in one-carbon unit metabolism as a one-carbon unit carrier

One-carbon units produced from amino acids can be converted into each other

The main function of the one-carbon unit is to participate in the synthesis of purine and pyrimidine

Methionine is involved in methyl transfer

Cysteine ​​is related to the production of various physiologically active substances

Phenylalanine → tyrosine

Tyrosine→Melanin Catecholamine

Catabolism of tryptophan produces pyruvate and acetoacetyl CoA