Isoenzymes

Isoenzymes  (isozymes)  are  various  forms  of  an  enzyme  that  catalyze  the similar reaction, but that exhibit several kinetic or physical properties,  like as  isoelectric   point,  pH  optimum,   substrate   affinity  or  effect  of  inhibitors. Several isoenzyme forms of a given enzyme are commonly derived from various genes and frequently occur in several tissues of the body.

An  instance  of  an  enzyme  that  has  many  isoenzyme  forms  is LDH   (lactate dehydrogenase)  that  catalyzes  the  reversible  conversion  of  pyruvate into  lactate  in  the  presence  of  the  coenzyme  NADH.  Lactate dehydrogenase is a tetramer of two various kinds of subunits that is called H and M, that have small

          Figure: The structures of the coenzymes FAD and FMN.

differences in amino  acid  sequence.  The two subunits can merge randomly with every other, forming five isoenzymes which have the compositions H₄, H₂M₂, H₃M, M₄ and HM₃. The five isoenzymes can be determined electrophoretically.  M  subunits  predominate  in  skeletal  muscle  and  liver  while  H subunits predominate  in the heart. H₃M  and H₄ isoenzymes  are mainly found predominantly  in  red  blood  cells  and the  heart ;  H₂M₂    is found  predominantly  in  the brain;  while  HM₃and  M₄ are  found  predominantly   in  the  liver  and  skeletal muscle.  Therefore, the isoenzyme pattern is characteristic of a particular tissue a factor which is of immense diagnostic importance in medicine. Infectious  hepatitis  myocardial infarction  and  muscle  diseases  includes  cell  death  of the affected  tissue  with  free  of  the  cell  contents  into  the  blood.  As lactate dehydrogenase is a soluble, cytosolic protein it is readily free in these situation. In the normal circumstances there is little lactate dehydrogenase in the blood.  Therefore the pattern of lactate dehydrogenase isoenzymes in the blood is indicative of the tissue that released the isoenzymes and so can be used to diagnose a condition and to monitor the progress of treatment.  Infarction  additionally to the pattern  of  lactate dehydrogenase isoenzymes for the clinical  diagnosis  of a myocardial, other enzymes,  including  creatine  kinase  and aspartate  aminotransferase are routinely  measured,  along  with  an ECG (electrocardiogram).