Primary alkaline phosphatase isoenzymes:
(1) liver
(2) bone
(3) placenta
(4) intestinal
(5) tumor-related (several types; Regan isoenzyme is placenta-like alkaline phosphatase, or PLAP; other forms include Nagao, Magoo, Kasahara, Timperley)
(6) other organs
Method for heat inactivation at 56°C (Moss, Henderson, 1994):
(1) Place 0.5 mL serum in a small test tube closed with Parafilm.
(2) Place the tube in a water bath preheated to 56°C. The water bath should maintain constant temperature during the incubation.
(3) After exactly 10 minutes quench the tube in ice water.
ratio of activity after to before heat exposure =
= (activity after heating to 56°C) / (activity unheated)
|
Activity Ratio
|
Likely Tissue of Origin
|
|
<= 20%
|
bone
|
|
21 – 55%
|
liver or intestinal
|
|
> 55%
|
placenta or Regan PLAP
|
where:
• Placenta or Regan PLAP activity are approximately 90% stable after heating.
Interpretation:
• Enzyme activity persisting after exposure to 65° C indicates placental or PLAP.
• The serum GGT will usually be elevated in patients with increased liver isoenzyme.
Limitations:
• Multiple isoenzymes may be elevated in patients with multisystemic disease (cancer with metastases to liver and/or bone, etc.).
• Reliable results require close attention to accurate temperature control and timing.
• Results are semi-quantitative.
• The method would be relatively insensitive to mild increases in the serum alkaline phosphatase activity, in which case isoenzyme electropheresis should be performed.
