"Selenium = Sulfur": Do you know what this mean???

The chemistry of selenium is similar to that od sulfur; consequently, selenium can often subsitute for sulfur.
The total body selenium contenet ranges from about 13 to 30mg.

       * 34Se                         " Structure of trigonal Selenium" 
                                                                
                                     
                                     
                                       +NH3
                                           :
"Selenomethionine"      HC--CH2--CH2--Se--CH3
                                           :
                                        COO-


                                      +NH3 
                                          :
"Selenocysteine"          HC--CH2--Se--H
                                          :
                                       COO-