lactose

see also:

• carbohydrates and sugars
• serum lactate and lactic acidosis
• lactose intolerance
• lactulose

• lactose is a disaccharide sugar which in nature, is only found in mammalian milk (mammals evolved 201-227 million years ago and one of the features in which they differed from sauropsids / reptilia was they had evolved to have milk producing breasts to feed their young)
• human milk contains 7% lactose by weight which is the highest concentration of all mammalian milks (cow's milk is 4-5% lactose)
• as lactose is soluble it is associated with the whey portion of milk and thus hard cheeses have neglible lactose
• lactose exists in the form of two anomers, α and β, the solubility in water of each changes significantly depending on temperature, at 20degC, the β anomer is 7x more soluble, while at 93.5degC, the α anomer becomes more soluble
  • when lactose is dried, a mixture of α-lactose, β-lactose, and α-lactose monohydrate is formed, the proportion of them varying according to the conditions of drying
  • α-lactose is highly hygroscopic, while the other forms are not, so caking and lumping tend to occur
  • anhydrous β-lactose, produced by crystallization of lactose above 93°C and roller drying, acquires the form of extremely fine crystals having high dissolution rate and good compactness, as its low moisture is adequate for moisture-sensitive active pharmaceutical ingredients, hence is often used as an excipient in pharmaceuticals
  • finely milled α-lactose monohydrate is used as excipient in dry powder inhalers such as those used for asthma
• lactose is barely sweet (about 15% sucrose), less soluble than its monosaccharide components, and less soluble than most sugars
• lactose is considered significantly less cariogenic than other sugars

• breast cells produce lactose from glucose and galactose absorbed from blood catalyzed by a protein complex called lactase synthase
  • the catalytic component, UDP-galactosyltransferase, catalyzes the transferring of the galactose moiety from UDP-galactose (formed via the Leloir pathway) to glucose
  • the noncatalytic component, α-lactalbumin, acts by reducing the Michaelis constant of the transferase by several hundredfold, thus giving glucose a definite advantage over other acceptors to be transgalactosylated ¹⁾
• control of milk production (lactation) is largely via prolactin and serotonin

• ingested lactose is broken down into glucose and galactose by lactase in the gut
  • duodenal villi secrete the enzyme lactase (β-D-galactosidase)
  • glucose and galactose can then be absorbed through the gut wall via transport mechanisms
• any lactose that is not broken down by gut lactase will generally be metabolized by the colonic microbiome or act as an osomotic agent and cause diarrhoea

• see lactose intolerance