This article summarizes my research over 40 years. The main theme of my work is nitrogen metabolism of amino acids, though later I focused on protein turnover in the cell. In the first years of my research work, I was busy dissecting the pathways involved in the metabolism of certain amino acids and their related enzymes. Then I became interested in the physiology and regulation of matabolism of these amino acids. For that, I used primary cultured hepatocytes, which contain many liver-specific enzymes. However, this play field was very rough around 1970 and hence I had to smooth them (differentiated) first. We discovered a specific growth factor (hepatocyte growth factor, HGF) in rat platelets. Exceptionally, I also worked on branched chain amino acids (valine, leucine and isoleucine). These amino acids are not efficiently metabolized in the liver, so I had to consider the physiology of extrahepatic tissues as well. Finally, I came across a huge protease complex, the proteasome. Whether these players, small amino acid metabolizing enzymes and the huge protease complex, danced well in harmony on my playground or not, I still do not know.

(Communicated by Tasuku HONJO, M.J.A.)