An alkaloid is an organic compound that contains nitrogen as the key atom that is produced by a plant. In potatoes and tomatoes, alkaloids are found as secondary metabolites (any substance produced or used during metabolism). It occurs as salts of plant acids such as malic, meconic, and quinic acids. Solanine in potato (Solanum tuberosum) and tomatine in tomato (Lycopersicum esculentum) are examples of plant alkaloids combined with sugar. Others occur as amides such as piperine from black pepper (Piper nigrum), Others occur as esters, such as, cocaine from coca leaves (Erythroxylum coca). Many of the bases or their salts are used medically. Caffeine, a central nervous system stimulant; is employed as a respiratory and cardiac stimulant and as an antidote to morphine and barbiturate poisoning. Colchicine, which is derived from the corms and seeds of the autumn crocus, is used as a gout suppressant. The key alkaloid of ipecac root (Cephaelis ipecacuanha), emetine, is used in the treatment of amebic dysentery and other protozoal infections.