5 - Glycoalkaloids, proteinase inhibitors and lectins

Summary

Although a valuable food, the potato has toxic, or potentially toxic, constituents: glycoalkaloids, proteinase inhibitors and lectins. These have been the subject of much research and debate, particularly in recent years. In this chapter, each toxic component group is reviewed, and its structure and probable function within the general physiology of the potato plant described briefly. Emphasis, however, is on the current consensus of opinions regarding nutritional and physiological significance of these components for human beings (for other reviews, see Jadhav & Salunkhe, 1975; Maga, 1980; Morris & Lee, 1984).

Glycoalkaloids

Chemical structure and content in the tuber

The Solanaceae family is recognized for the numerous alkaloids found among its various member species. Alkaloids are nitrogen-containing organic compounds occurring in plants, as well as in a small number of animal products (Robinson, 1974). As a result of the diverse pharmacological properties of alkaloids, many plants have long been used as drug sources; some were prescribed for their curative or beneficial effects; many others have become well known for their poisonous, aphrodisiac, narcotic or hallucinogenic attributes.

Two such narcotic and hallucinogenic plants, the mandrake and deadly nightshade, are related to the potato. When first introduced into Europe, potatoes may have been shunned because of their ‘guilty association’ with such notorious relatives (Rhoades, 1982). Under normal conditions of human consumption, the amounts of potato alkaloids ingested are not harmful. Sometimes, however, alkaloid quantities can increase to toxic, and in rare instances fatal, levels. It is necessary, therefore, to understand how such toxic levels arise and what can be done to prevent them.