Specialised metabolism

Many cells, and therefore tissues of the body, have the common pathways of catabolism and anabolism described in Chapters 16 and 17. However, some tissues have specific metabolic pathways that allow those tissues to fulfil their role. The liver, for example, has the complete gluconeogenic pathway allowing it to control blood glucose levels and the β-cells of the pancreas ‘tune’ their metabolism to release insulin in response to an increase in blood glucose.

This chapter looks at three more examples of specialised metabolism: how differences in carbohydrate metabolism in the cells of the mammary gland produce lactose, how differences in amino acid metabolism in the central nervous system (CNS) produce and recycle amino-acid-based neurotransmitters, and how transport systems in the parietal cells of the stomach can produce hydrochloric acid.

Lactose production in the mammary gland

Chapter 15 details how the body deals with lactose that is ingested as part of the diet so that the body can access the energy in this carbohydrate. However, mammals also produce lactose as part of the milk secreted from the mammary glands.

When a woman becomes pregnant, small sacs in her mammary glands, known as alveoli, form and are lined by alveolar epithelial cells. These epithelial cells secrete lactose into the alveolus along with a number of milk proteins such as casein and lactalbumin. The synthesis of lactose ‘hijacks’ parts of other pathways.

Lactose is a disaccharide of glucose and galactose. Glucose is readily available and some galactose can be obtained from the diet. However, if needed, the alveolar epithelial cells can synthesise galactose. Galactose is an epimer of glucose, differing only in the arrangement of the atoms around the C₄-carbon (Fig 19-1).