Sentence:  They were then centrifuged at 59780 rpm for 35 to 80 min at 20 ` C in a Spinco model E ultracentrifuge at a <WSD>protein</WSD> concentration of 1.00 to 1.25 % . 
Meaning of the word marked with <WSD> tags:  any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes