Born in Rostock, Germany, Kossel was professor of physiology at the universities of Marburg and Heidelberg (1895–1923). He had first studied medicine, but turned his attention to biochemistry under the influence of Felix Hoppe-Seyler, whose assistant he was at Strasbourg (1877–81). Kossel was also for a time a colleague of Emil Du Bois-Reymond. While with Hoppe-Seyler, Kossel continued the latter's investigations of the cell substance called nuclein, demonstrating that it contained both protein and nonprotein (nucleic acid) parts. He was further able to show that the nucleic acids, when broken down, produced nitrogen-bearing compounds (purines and pyrimidines) as well as carbohydrates. Kossel also studied the proteins in spermatozoa, being the first to isolate the amino acid histidine. He was awarded the Nobel Prize for physiology or medicine in 1910 for his work on cells and proteins.