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• Aspartate aminotransferase(AST): This is one of a number of transaminases involved
  in intermediary metabolism. It is found in most tissues but is abundant in heart and liver.
  Its activity in serum is raised following a myocardial infarction and reaches a peak
  between 48 and 60 hours. It has little clinical value in the early diagnosis of heart muscle
  damage but is of use in the case of delayed presentation with chest pain.


• Lactate dehydrogenase(LD): This is a tetrameric protein made of two monomers
  denoted as H (heart) and M (muscle) so that five isoforms exist: LD-1 (H4), LD-2 (H3M),
  LD-3 (H2M2), LD-4 (HM3) and LD-5 (M4). LD-1 predominates in heart, brain and kidney
  and LD-5 in skeletal muscle and liver. Total LD activity and LD-1 activity in serum
  increases following a myocardial infarction and reaches a peak after 48–72 hours.
  The subsequent decline in activity is much slower than that of CK or AST. The
  diagnostic value of LD activity measurement is mainly confined to monitoring the
  patient’s recovery from the myocardial infarction event.

Typical changes in the activities of these three enzymes following a myocardial

infarction are shown in Fig. 16.4. All three enzymes are assayed by an automated method

based on the following reactions.

• Total CK activity: This is assessed by coupled reactions (Section 15.3.2) with hexokinase
  and glucose-6-phosphate dehydrogenase in the presence ofN-acetylcysteine as

CK-MB

Total CK

LD1

AST

Troponin I

Myoglobin

024681012

Time from onset of chest pain (days)

Change in activity/concentration

Fig. 16.4Serum enzyme activity and myoglobin and troponin-I concentration changes following a myocardial

infarction. Changes are expressed as a multiple of the upper limit of the reference range. Values vary according

to the severity of the event, but the time course of each profile is characteristic of all events.

643 16.3 Examples of biochemical aids to clinical diagnosis