Serine, threonine, tyrosine and derivatives

These 3 amino acids are often partially destroyed by acid hydrolysis with the amount of destruction dependent on the time of hydrolysis. With some proteins there is little or no loss of threonine or tyrosine, but serine destruction is rather constant in all proteins. It has been common practice to correct serine values by 10% for each 24 hr of acid hydrolysis, and sometimes threonine and tyrosine values are corrected by 5 % as a first approximation of the extent of destruction. 

The most accurate method of analysis for serine, threonine and tyrosine in proteins is to hydrolyze the proteins for several different times (e.g. 24, 48 and 72 hr) and extrapolate the values to 0 time. In general, the more times that are utilized, the more accurate will be the extrapolation. Extrapolation of the NHj content of these hydrolysates to 0 time will also give an estimate of the number of amides present in the protein, if care has been used to exclude NHj during purification of the protein. The inclusion of phenol in the hydrolysates has generally decreased the rates of destruction, particularly for tyrosine. If serine phosphate ( 2.12.4) or similar derivatives are present, the extrapolated curve will be more complex due to different rates of destruction. 

Since most derivatives of serine and threonine are not stable to acid hydrolysis, they will not be discussed here. However, those occurring naturally in proteins are described in 2.12.4. It should be noted that 0-maleylated derivatives and similar derivatives of serine and threonine have been found as side-products of reactions used to modify other residues in proteins (e.g. ch. 3). 

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