以下是一篇关于Neu5Gc的文章。我不是这专业的，能猜个大概。基本意思是：人跟其它哺乳动物不同；其它哺乳动物有产生Neu5Gc的基因，而人没有。人有的是产生Neu5Ac的基因。Neu5Ac 与Neu5Gc就差一个氧原子。如果Neu5Gc通过哺乳动物的肉食（如猪肉牛肉）进入人体，就会产生自免疫反应，从而导致身体的低度炎症。很多心脏病、癌症都从此而来。

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N-Glycolylneuraminic acid (Neu5Gc) is a sialic acid molecule found in most mammals.

SIALIC ACID = from silicates and aluminium minerals.

Humans cannot synthesize (use, consume, get nutrient out of, or more importantly to eliminate)  Neu5Gc because the human gene CMAH is irreversibly mutated, though it is found in apes.

It is absent in human tissues because of the inactivation of gene encoding CMP-N-acetylneuraminic acid hydroxylase.

Neu5Gc can be found in mammals (most of them vegan animals) , but trace amounts can be found in humans, even though the gene to encode for production of Neu5Gc was eliminated long ago. These trace amounts of Neu5Gc come from the consumption of mammals in the human diet.

Even though Neu5Gc cannot be produced by humans, it is still reported to be found in human cancers and fetal samples. This suggests that these Neu5Gc molecules must enter the human pathways through external sources, like through diets.

As found in the observation of cultured human cells, the cells expressed Neu5Gc due to their uptake of animal products in the medium. By macropinocytosis, (absorbed through cell walls) the sialic acid can be transferred to the cytosol (fluid inside a skin cell) by a sialin (protein) transporter.

Due to the fact that Neu5Gc (from mammals) differs from the human Neu5Ac by only one oxygen atom, cells will uptake the molecule as if it were native to the cell.

Although the biochemical pathways don’t recognize this molecule (human Neu5Ac) as foreign the human immune system does, which can bring about many problems.

Epidemiological studies have shown a correlation between mammal meat consumption and increased risks of many diseases. These diseases include carcinomas, atherosclerosis, and type-2 diabetes. The uptake of Neu5Gc from this diet aggravates these diseases.

Because of the anti-Neu5Gc antibodies that the human body carries, ingestion of Neu5Gc causes chronic inflammation.

In recent findings, it has been discovered that all humans have Neu5Gc-specific antibodies, often at high levels (antibodies are fighting the Neu5Gc meat consumption). 

Sialic acids are negatively charged and hydrophillic, so they don’t readily cross the lipid, hydrophobic membranes of cells. It is because of this that the uptake of Neu5Gc must occur through an endocytic pathway. (absorbed directly through cell walls including skin) More specifically, exogenous Neu5Gc molecules enter cells through clathrin-independent endocytic pathways with help from pinocytosis. 

After the Neu5Gc has entered the cell via pinocytosis, the molecule is released by lysosomal sialidase. The molecule is then transferred into the cytosol by the lysosomal sialic acid transporter. From here, Neu5Gc are available for activation and addition to glycoconjugates. Because Neu5Gc appears to be enhanced in naturally occurring tumors and fetal tumors, it is suggested that this uptake mechanism is enhanced by growth factors. (growth hormones) 

THE MUTILATED HUMAN GENE

CMAH = Cytidine monophospho-N-acetylneuraminic acid hydroxylase,

pseudogene Identifiers
Symbols CMAHP; CMAH; CSAH

Putative cytidine monophosphate-N-acetylneuraminic acid hydroxylase-like protein is an enzyme that in humans is encoded by the CMAH gene.

Sialic acids are terminal components of the carbohydrate chains of glycoconjugates involved in ligand–receptor, cell–cell, and cell–pathogen interactions.

The two most common forms of sialic acid found in mammalian cells are N-acetylneuraminic acid (Neu5Ac) and its hydroxylated derivative, N-glycolylneuraminic acid (Neu5Gc-found in mammals except humans).

Studies of sialic acid distribution show that Neu5Gc is not detectable in normal human tissues although it was an abundant sialic acid in other mammals. Neu5Gc is, in actuality, immunogenic (produces a immune response) in humans.

The absence of Neu5Gc in humans is due to a deletion within the human gene CMAH encoding cytidine monophosphate-N-acetylneuraminic acid hydroxylase, an enzyme responsible for Neu5Gc biosynthesis.

Sequences encoding the mouse, pig, and chimpanzee hydroxylase enzymes were obtained by cDNA cloning and found to be highly homologous. However, the homologous human cDNA differs from these cDNAs by a 92-bp deletion in the 5' region. This deletion, corresponding to exon 5 of the mouse hydroxylase gene, causes a frameshift mutation and premature termination of the polypeptide chain in human.

It seems unlikely that the truncated human hydroxylase mRNA encodes for an active enzyme explaining why Neu5Gc is undetectable in normal human tissues.

The deletion that deactivated this gene occurred approximately 3.2 mya, after the divergence of humans from the African great apes, and quickly swept to fixation in the human population. The lineage of this pseudogene in humans indicates another deep split in Africa dating to 2.9 Mya, with a complex subsequent history.