Total homocysteine is defined as the sum of all homocysteine species in plasma/serum, including free and protein-bound forms. In the present review, we compare and evaluate several techniques for the determination of total homocysteine. Because these assays include the conversion of all forms into a single species by reduction, the redistribution between free and protein-bound homocysteine through disulfide interchange does not affect the results, and total homocysteine can be measured in stored samples. Total homocysteine in whole blood increases at room temperature because of a continuous production and release of homocysteine from blood cells, but artificial increase is low if the blood sample is centrifuged within 1 h of collection or placed on ice. Different methods correlate well, and values between 5 and 15 mumol/L in fasting subjects are considered normal. Total homocysteine in serum/plasma is increased markedly in patients with cobalamin or folate deficiency, and decreases only when they are treated with the deficient vitamin. Total homocysteine is therefore of value for the diagnosis and follow-up of these deficiency states and may compensate for weaknesses of the traditional laboratory tests. In addition, total homocysteine is an independent risk factor for premature cardiovascular diseases. These disorders justify introduction of the total homocysteine assay in the routine clinical chemistry laboratory.

https://www.folk.uib.no/mfapu/Pages/papers%20pdf/1993/ueland_2000_cc_39_1764.pdf

Total homocysteine in plasma or serum: methods and clinical applications.

https://www.cell.com/trends/neurosciences/pdf/S0166-2236(03)00032-8.pdf

Folate and homocysteine metabolism in neural plasticity and neurodegenerative disorders.

The ability to take up substances from the surrounding environment not only provides cells with vital nutrients, but also enables the selective transport of substances from one compartment to another. Megalin and cubilin are two structurally different endocytic receptors that interact to serve such functions. Evidence has accumulated in recent years to indicate that these receptors have important functions in both normal physiology and pathology.

Megalin and cubilin: multifunctional endocytic receptors

Nutritional and physiologic significance of human milk proteins

Rapid high-performance liquid chromatographic assay for total homocysteine levels in human serum.

The dynamics of transcobalamin ii. a vitamin b12 binding substance in plasma.

Inherited selective intestinal malabsorption of cobalamin (Cbl) was observed in a family of giant schnauzer dogs. Family studies and breeding experiments demonstrated simple autosomal recessive inheritance of this disease. Affected puppies exhibited chronic inappetence and failure to thrive beginning between 6 and 12 wk of age. Neutropenia with hypersegmentation, anemia with anisocytosis and poikilocytosis, and megaloblastic changes of the bone marrow were present. Serum Cbl concentrations were low, and methylmalonic aciduria and homocysteinemia were present. Parenteral, but not oral, cyanocobalamin administration rapidly eliminated all signs of Cbl deficiency except for low serum Cbl concentrations. Cbl malabsorption in affected dogs was documented by oral administration of [57Co]cyanocobalamin with or without simultaneous oral administration of intrinsic factor or normal dog gastric juice. Quantitation and function studies of intrinsic factor and transcobalamin-II from affected dogs revealed no abnormality. Other gastrointestinal functions and ileal morphology were normal, indicating a selective defect of Cbl absorption at the level of the ileal enterocyte. Immunoelectron microscopy of ileal biopsies showed that the receptor for intrinsic factor-Cbl complex was absent from the apical brush border microvillus pits of affected dogs. This canine disorder resembles inherited selective intestinal Cbl malabsorption (Imerslund-Grasbeck syndrome) in humans, and is a spontaneously occurring animal model of early onset Cbl deficiency.

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Inherited selective intestinal cobalamin malabsorption and cobalamin deficiency in dogs.

The 13 cases of methylcobalamin (MeCbl) deficiency presenting in early infancy have all been developmentally delayed, and the majority have had seizures, hypotonia, lethargy, and microcephaly. The CNS injury appears to occur during the first 6 months of postnatal life. The same symptoms are seen in acquired cobalamin (Cbl) deficiency in the same age group. MRI performed at age 18-19 months and after 13-14 months of large amounts of Cbl, in two cases showed delayed myelination, most pronounced in the cerebrum. Isolated MeCbl deficiency is the consequence of cblE and G mutations where the lesion is of a single Cbl-dependent enzyme, the methyltransferase. One effect of a deficiency of MeCbl, and of the associated failure of the methionine synthase reaction, is, therefore, an impairment of myelination of the brain of the newborn. The slow, but usually incomplete, improvement in psychomotor status after years of treatment with Cbl may be related to the eventual myelination. However, the hypotonia, lethargy, and impaired responsiveness react to treatment with Cbl within 24-48 hours, which suggests an expression of MeCbl deficiency on the CNS distinct from the delayed myelination. Although there is much to be learned, it is now clear that a normally functioning Cbl-dependent methyl transferase is required for development and function of the human brain.

Function of vitamin B12 in the central nervous system as revealed by congenital defects

The content, binding, and forms of vitamin B12 in milk.

Presented is a modification of an assay for total serum homocysteine (Hcy) in which the Hcy plus radioactive adenosine is converted enzymatically to labeled S-adenosylhomocysteine (AdoHcy). The modifications included a commerical source for the AdoHcy hydrolase, adenosine labeled with either 14C or 3H, and separation of the AdoHcy by thin layer chromatography. The assay was sensitive to 25 pmol. Hcy levels in sera from 18 controls ranged from 6.9 to 12.1 mumol/L with a mean of 9.1 and a SD of 1.5 mumol/L. The total serum Hcy was increased in vitamin B12 and folate deficiency. The level was high in congenital defects of vitamin B12 metabolism, blocking the methylation of Hcy regardless of the serum vitamin B12 levels, but was normal in the absence of tissue deficiency even if the serum vitamin B12 levels were low. The procedure has been found practical in two years of use and requires only 0.1 mL of serum.

Charles A. Hall

Papers

The dynamics of transcobalamin ii. a vitamin b12 binding substance in plasma.

Inherited selective intestinal cobalamin malabsorption and cobalamin deficiency in dogs.

Function of vitamin B12 in the central nervous system as revealed by congenital defects

The content, binding, and forms of vitamin B12 in milk.

The total serum homocysteine as an indicator of vitamin B12 and folate status.