Selasa, 23 Oktober 2012 - 20:58:05 WIB
Metabolisme Bilirubin
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Kategori: Gastroentero Hepatology - Dibaca: 3156 kali

Infopenyakitdalam.com - Bilirubin adalah suatu pigmen berwarna kuning berasal dari unsur porfirin dalam hemoglobin yang terbentuk sebagai akibat penghancuran sel darah merah oleh sel-sel retikuloendotel.

Sungguhpun berasal dari hemoglobin, bilirubin tidak mengandung zat besi. Bilirubin yang bau terbentuk ini larut dalam lemak. Di dalam plasma darah bilirubin ini berikatan dengan albumin. Karena terbentuk secara normal dari penghancuran sel darah merah maka proses metabolisme dan sekresi selanjutnya dapat berlangsung secara terus menerus.

Hemoglobin yang berasal dari penghancuran eritrosit oleh makrofag di dalam limfa, hati dan alat retikuloendotel lain akan mengalami proses pemecahan menjadi heme dan globin. Melalui proses oksidasi, komponen globin mengalami degadrasi menjadi asam amino dan digunakan untuk pembentukan protein lain.

Unsur heme selanjutnya  oleh heme-oksigenase, teroksidasi menjadi biliverdin dengan melepas zat besi dan karbonmonoksida. Biliverdin redukse akan mereduksi biliverdin menjadi bilirubin tidak terkonjugasi.

Sungguhpun lebih dari 80% bilirubin terjadi dari pemecahan heme yang berasal dari eritrosit namun sekitar 15-20% bilirubin dapat pula berasal dari hemoprotein lain seperti mioglobin, sitokrom. Bilirubin tidak terkonjugasi ini adalah suatu zat lipofilik, larut dalam lemak, hampir tidak larut dalam air sehingga tidak dapat dikeluarkan dalam urin melalui ginjal (disebut pula bilirubin indirek karena hanya bereaksi positif pada tes setelah dilarutkan dalam alkohol). Karena sifat lippofilik, zat ini dapat melalui membran sel dengan relatif mudah. Setelah dilepas ke dalam plasma sebagian besar bilirubin tidak terkonjugasi ini membentuk ikatan dengan albumin sehingga dapat larut di dalam darah. Pigmen ini secara bertahap berdifusi ke dalam sel hati (hepatosit). Dalam hepatosit, bilirubin tak berkonjugasi, dikonjugasi dengan asam glukoromat membentuk bilirubin glukuronida atau bilirubin terkonjugasi (disebut pula bilirubin direk). Reaksi konjugasi dikatalisasi oleh enzim glukoronil transferase suatu enzim yang terdapat di retikulum endoplasmik dan merupakan kelompok enzim yang mampu memodifikasi zat asing yang bersifat toksik. Kelompok enzim ini dapat diaktifkan dengan rangsangan fenobarbital, oleh karena itu fenobarbital dapat dijadikan sebagai pengobatan, terutama apabila hanya terjadi penurunan kadar glukonil trasferase.

Bilirubin terkonjugasi larut dalam air, dapat dikeluarkan melalui ginjal namun dalam keadaan normal tidak dapat terdeteksi dalam urin. Sebagian besar bilirubin terkonjugasi ini dikeluarkan ke dalam empedu, suatu campuran kolesterol, fospholipid, bilirubin glukuronida dan garam empedu. Sesudah dilepas ke dalam salurann cerna bilirubin glukoronida (bilirubin terkonjugasi) diaktifkan oleh enzim bakteri di dalam usus, sebagian menjadi komponen urobilinogen yang akan keluar dalam tinja (sterkobilin), atau diserap kembali dari saluran cerna, dibawa ke hati dan dikeluarkan kembali ke dalam empedu. Urobilinogen dapat larut dalam air, oleh karena itu sebagian dikeluarkan melalui ginjal.

Sumber: Mpu Kanoko, dalam Ilmu Ajar Ilmu Penyakit Hati, Jaya Abadi, Jakarta, 2007





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